  United States Court of Appeals
      for the Federal Circuit
                ______________________

     TEVA PHARMACEUTICALS USA, INC.,
 TEVA PHARMACEUTICAL INDUSTRIES, LTD.,
        TEVA NEUROSCIENCE, INC.,
YEDA RESEARCH AND DEVELOPMENT CO., LTD.,
             Plaintiffs-Appellees

                           v.

 SANDOZ, INC., MOMENTA PHARMACEUTICALS
                    INC.,
             Defendants-Appellants

 MYLAN PHARMACEUTICALS INC., MYLAN INC.,
          NATCO PHARMA LTD.,
           Defendants-Appellants

        SANDOZ INTERNATIONAL GMBH,
                NOVARTIS AG,
                   Defendants
             ______________________

      2012-1567, 2012-1568, 2012-1569, 2012-1570
               ______________________

   Appeals from the United States District Court for the
Southern District of New York in Nos. 08-CV-7611, 09-
CV-8824, Judge Barbara S. Jones.
                ______________________

                Decided: June 18, 2015
                ______________________
2                  TEVA PHARMACEUTICALS USA   v. SANDOZ INC.


   ELIZABETH HOLLAND, Goodwin Procter LLP, New
York, NY, for plaintiffs-appellees. Also represented by
DAVID M. HASHMALL; JOHN T. BENNETT, HENRY C. DINGER,
JOHN C. ENGLANDER, NICHOLAS K. MITROKOSTAS, DARYL L.
WIESEN, Boston, MA; WILLIAM G. JAMES, II, WILLIAM M.
JAY, Washington, DC.

    DEANNE MAYNARD, Morrison & Foerster LLP, Wash-
ington, DC, for defendants-appellants Sandoz, Inc., Mo-
menta Pharmaceuticals Inc. Also represented by MARC A.
HEARRON, BRIAN ROBERT MATSUI; ANDERS T. AANNESTAD,
DAVID CLARENCE DOYLE, BRIAN M. KRAMER, San Diego,
CA.

    SHANNON BLOODWORTH, Perkins Coie, LLP, Washing-
ton, DC, for defendants-appellants Mylan Pharmaceuti-
cals Inc., Mylan Inc., Natco Pharma Ltd.        Also
represented by BRANDON MICHAEL WHITE; DAVID LEE
ANSTAETT, DAVID E. JONES, Madison, WI; EVAN CHESLER,
Cravath Swaine & Moore LLP, New York, NY.
                ______________________

    Before MOORE, MAYER, and WALLACH, Circuit Judges. 1
     Opinion for the court filed by Circuit Judge MOORE.
      Dissenting opinion filed by Circuit Judge MAYER.
MOORE, Circuit Judge.
   This case comes to us on remand from the Supreme
Court, which vacated our earlier opinion reversing the


     1 Pursuant to Fed. Cir. Internal Operating Proce-
dure 15 ¶ 2(b)(ii), Circuit Judges Mayer and Wallach were
designated to replace Circuit Judge Randall R. Rader,
now retired, and District Judge Dee V. Benson, United
States District Court for the District of Utah.
TEVA PHARMACEUTICALS USA   v. SANDOZ INC.                3


district court’s judgment that certain claims were not
indefinite (Group I claims), and affirming the district
court’s holdings that other claims (Group II claims) were
valid and infringed. 2 Teva Pharm. USA, Inc. v. Sandoz,
Inc., 723 F.3d 1363 (Fed. Cir. 2013), vacated, Teva Pharm.
USA, Inc. v. Sandoz, Inc., 135 S. Ct. 831 (2015). Relevant
to the Group I claims, the Supreme Court held that the
ultimate construction of a claim term is a question of law,
subject to de novo review, and that underlying subsidiary
fact findings are subject to clear error review. Teva, 135
S. Ct. at 837–38, 841–42. During that same time, the
Supreme Court issued its opinion in Nautilus, Inc. v.
Biosig Instruments, Inc. (Nautilus II), 134 S. Ct. 2120
(2014), addressing the standard for indefiniteness. On
remand, the parties submitted supplemental briefing
explaining how the appeal should be resolved in light of
the Supreme Court’s Teva decision. See Teva Pharm.
USA, Inc. v. Sandoz, Inc., No. 12-1567 (Fed. Cir. Feb. 20,
2015), ECF No. 137. Applying the legal standards set
forth in the Supreme Court’s Teva and Nautilus II deci-
sions, we hold that the Group I claims are invalid for
indefiniteness. 3


   2    The asserted patents were: U.S. Patent Nos.
5,800,808; 5,981,589; 6,048,898; 6,054,430; 6,342,476;
6,362,161; 6,620,847; 6,939,539; and 7,199,098.       The
Supreme Court decision does not affect our prior holding
with respect to the Group II claims. For the reasons
articulated in our earlier opinion, we adopt those hold-
ings.
    3   While the case was pending at the Supreme
Court, all of the patents-in-suit expired, with the excep-
tion of U.S. Patent No. 5,800,808. Thus, claim 1 of the
’808 patent is the sole unexpired Group I claim. Our
analysis will therefore focus on that claim, but to the
extent that issues relating to the expired Group I claims
4                 TEVA PHARMACEUTICALS USA    v. SANDOZ INC.


                       BACKGROUND
    The facts of this case were recited in this court’s pre-
vious opinion. In summary, Appellants submitted Abbre-
viated New Drug Applications (ANDAs) to the Food and
Drug Administration (FDA) seeking approval to market
generic versions of Copaxone®. Teva, which markets
Copaxone®, sued Appellants for patent infringement
under 35 U.S.C. § 271(e)(2)(A). Claim 1 of the ’808 patent
recites a method of making a product called copolymer-1:
    A method of manufacturing copolymer-1, compris-
    ing reacting protected copolymer-1 with hydro-
    bromic acid to form trifluoroacetyl copolymer-1,
    treating said trifluoroacetyl copolymer-1 with
    aqueous piperidine solution to form copolymer-1,
    and purifying said copolymer-1, to result in copol-
    ymer-1 having a molecular weight of about 5 to 9
    kilodaltons.
’808 patent claim 1 (emphases added).
     Copolymer–1 consists of four different amino acids
(alanine, glutamic acid, lysine, and tyrosine) combined in
a certain ratio to make a polypeptide product. A sample
of polymeric material like copolymer–1 typically consists
of a mixture of individual polymer molecules that have
varying molecular weights. There are three different
measures of molecular weight relevant to this appeal:
peak average molecular weight (Mp), number average
molecular weight (Mn), and weight average molecular
weight (Mw). Each measure is calculated in a different
manner. The claim does not specify which measure to use
and in a typical polymer sample, Mp, Mn, and Mw have
different values.


remain unresolved, this analysis should be understood to
apply equally to the other Group I claims.
TEVA PHARMACEUTICALS USA   v. SANDOZ INC.                 5


     The district court rejected the Appellants’ argument
that the term “molecular weight” was indefinite. Teva
Pharm. USA, Inc. v. Sandoz, Inc. (Markman Order), 810
F. Supp. 2d 578, 586–93, 596 (S.D.N.Y. 2011). The dis-
trict court found credible Dr. Grant’s testimony that Mp is
the only type of average molecular weight that can be
directly obtained from a chromatogram and calibration
curve obtained by the analytical method described in
Example 1 (Size Exclusion Chromatography or SEC). Id.
at 588, 590. It noted that experts testified that Mn and
Mw can be obtained from the chromatogram and calibra-
tion curve, but doing so would require additional data
manipulation and calculations not disclosed in the specifi-
cation. Id. It therefore credited Dr. Grant’s opinion that
Example 1 implies the use of Mp. Id. The district court
also found that Example 1 corresponds to Figure 1 in the
patent specification. Id. at 588. It considered Appellants’
argument that Figure 1 does not disclose Mp because the
peaks of the depicted curves do not match the molecular
weight values reported in the legend. Id. at 590. The
district court, however, accepted Dr. Grant’s opinion that
a person of ordinary skill would understand that the
process of transferring data from a chromatogram could
cause a shift in the peak of the curves. Id. It therefore
concluded that the fact that the peaks do not match the
listed molecular weights does not dissuade the conclusion
that “molecular weight” means Mp. Id. at 590–91. The
district court determined that “the prosecution history
also indicates [average molecular weight] refers to Mp in
the context of the patents-in-suit.” Id. at 589. It rejected
as irrelevant the patentee’s response to an indefiniteness
rejection during the prosecution of the ’847 patent that
“[o]ne of ordinary skill in the art could understand that
kilodalton unit implies a weight average molecular
weight.” Id. at 591–92. It did so on the basis that the
“statement was incorrect” because each type of average
6                 TEVA PHARMACEUTICALS USA    v. SANDOZ INC.


molecular weight can use the kilodalton. Id. at 592. It
then concluded that one of ordinary skill would accept the
patentee’s statement during the prosecution of the ’539
patent that average molecular weight means Mp. Id.
Having considered the claims, specification, prosecution
history and extrinsic evidence, the district court deter-
mined that “molecular weight” means Mp in the context of
the claimed invention and held that the claims are not
indefinite. Id.
    We reversed the district court’s judgment with respect
to the Group I claims, holding them indefinite. Teva, 723
F.3d at 1368–69. Teva filed a petition for a writ of certio-
rari, arguing to the Supreme Court that in holding the
claims indefinite, we erred by giving no weight to the
district court fact findings. Pet. for Writ of Cert., Teva
Pharm. USA, Inc. v. Sandoz, Inc., No. 13-854, 2014 WL
230926, at *13–14 (2014). Teva argued that this court’s
determinations in Cybor Corp. v. FAS Techs., Inc., 138
F.3d 1448 (Fed. Cir. 1998) (en banc) and Lighting Ballast
Control LLC v. Philips Elecs. North America Corp., 744
F.3d 1272 (Fed. Cir. 2014) (en banc) that we review all
aspects of claim construction de novo was incorrect and
inconsistent with Fed. R. Civ. P. 52(a)(6). Brief for the
Petitioner at 18, Teva Pharm. USA, Inc. v. Sandoz, Inc.,
No. 13-854 (2014). It argued that under Fed. R. Civ. P.
52(a)(6), we should only set aside a district court fact
finding if such finding is clearly erroneous. The Supreme
Court agreed.
    The Supreme Court held that “it was proper to treat
the ultimate question of the proper construction of the
patent as a question of law in the way that we treat
document construction as a question of law.” Teva, 135 S.
Ct. at 837. The reviewing court, however, should review
subsidiary factual findings under the clearly erroneous
standard. Id. at 838. The Court explained that “when the
TEVA PHARMACEUTICALS USA    v. SANDOZ INC.                 7


district court reviews only evidence intrinsic to the patent
(the patent claims and specifications, along with the
patent’s prosecution history), the judge’s determination
will amount solely to a determination of law” which will
be reviewed de novo. Id. at 841. If a district court needs
to consult extrinsic evidence, for example, to understand
the meaning of a term in the relevant art at the relevant
time, the court may need to make subsidiary factual
findings about that extrinsic evidence. Id. The Court
explained that
    if a district court resolves a dispute between ex-
    perts and makes a factual finding that, in general,
    a certain term of art had a particular meaning to
    a person of ordinary skill in the art at the time of
    the invention, the district court must then conduct
    a legal analysis: whether a skilled artisan would
    ascribe that same meaning to that term in the
    context of the specific patent claim under review.
Id. Experts may explain terms of art and the state of the
art at any given time, but they cannot be used to prove
the legal construction of a writing. Id. If a district court
resolves a subsidiary factual dispute, it will then interpret
the patent claim in light of the facts as the court found
them. Id. As the Court cautioned, an “issue does not lose
its factual character merely because its resolution is
dispositive of the ultimate legal question.” Id. at 842
(citations and quotation marks omitted). Regardless of
whether a subsidiary factual finding plays a small or
large role in the ultimate conclusion about the meaning of
the patent term, “the ultimate question of construction
will remain a legal question.” Id. at 841–42.
    The Court vacated our decision, concluding that Teva
identified at least one factual finding by the district court
which we did not review for clear error. Appellants had
argued that “molecular weight” could not mean Mp be-
8                 TEVA PHARMACEUTICALS USA    v. SANDOZ INC.


cause the molecular weight values in the Figure 1 legend
did not match up with the peak values on the Figure 1
curves. The Supreme Court concluded that the district
court’s finding “about how a skilled artisan would under-
stand the way in which a curve created from chromato-
gram data reflects molecular weights” was a factual
finding. Id. at 843. Distinguishing between the factual
and legal components of the analysis, the Court explained
that “[b]ased on that factual finding, the District Court
reached the legal conclusion that figure 1 did not under-
mine Teva’s argument that molecular weight referred to
the first method of calculation (peak average molecular
weight).” Id. The Court vacated our decision instructing
that the district court fact findings should be reviewed for
clear error. Id. The Court acknowledged that Teva
claimed there were two additional instances in which the
Federal Circuit rejected fact findings without finding
clear error. Expressing no opinion on those arguments,
the Court left these matters for us to consider. Id.
    While Teva was pending at the Supreme Court, the
Court issued its opinion in Nautilus II. In Nautilus II,
the Court evaluated our standards for indefiniteness
under 35 U.S.C. § 112, ¶ 2 (2006), 4 rejecting our “not
amenable to construction or insolubly ambiguous” stand-
ard. Those standards were the ones applied in our Teva
decision. The Supreme Court articulated the standard to
be applied: “[W]e hold that a patent is invalid for indefi-
niteness if its claims, read in light of the specification
delineating the patent, and the prosecution history, fail to

    4   Paragraph 2 of 35 U.S.C. § 112 was replaced with
newly designated § 112(b) when § 4(c) of the Leahy–Smith
America Invents Act (“AIA”), Pub. L. No. 112–29, took
effect on September 16, 2012. Because this case was filed
before that date, we will refer to the pre-AIA version of §
112.
TEVA PHARMACEUTICALS USA    v. SANDOZ INC.                 9


inform, with reasonable certainty, those skilled in the art
about the scope of the invention.” Nautilus II, 134 S. Ct.
at 2124 (emphasis added). As we explained on remand,
“[t]he Court has accordingly modified the standard by
which lower courts examine allegedly ambiguous claims;
we may now steer by the bright star of ‘reasonable cer-
tainty,’ rather than the unreliable compass of ‘insoluble
ambiguity.’” Biosig Instruments, Inc. v. Nautilus, Inc.,
No. 2012-1289, slip op. at 8 (Fed. Cir. Apr. 27, 2015).
    We therefore reconsider the district court’s claim con-
struction and indefiniteness determination in light of the
Supreme Court’s guidance.
                        DISCUSSION
    A patent’s specification must “conclude with one or
more claims particularly pointing out and distinctly
claiming the subject matter which the applicant regards
as [the] invention” 35 U.S.C. § 112, ¶ 2. A patent is
indefinite “if its claims, read in light of the specification
delineating the patent, and the prosecution history, fail to
inform, with reasonable certainty, those skilled in the art
about the scope of the invention.” Nautilus II, 134 S. Ct.
at 2124. The definiteness requirement must take into
account the inherent limitations of language. “Some
modicum of uncertainty . . . is the ‘price of ensuring the
appropriate incentives for innovation.’” Id. at 2128 (quot-
ing Festo Corp. v. Shoketsu Kinzoku Kogyo Kabushiki Co.,
535 U.S. 722, 732 (2002)). On the other hand, “a patent
must be precise enough to afford clear notice of what is
claimed, thereby appris[ing] the public of what is still
open to them.” Id. at 2129 (internal quotation marks and
citations omitted). Indefiniteness is a question of law that
we review de novo. Wellman, Inc. v. Eastman Chem. Co.,
642 F.3d 1355, 1365–66 (Fed. Cir. 2011).
10                 TEVA PHARMACEUTICALS USA    v. SANDOZ INC.


    To determine whether the Group I claim at issue is
indefinite, we look to the patent record—the claims,
specification, and prosecution history—to ascertain if they
convey to one of skill in the art with reasonable certainty
the scope of the invention claimed. We conclude in this
case that they do not convey with reasonable certainty the
measure of molecular weight to be used. Claim 1 of the
’808 patent recites “molecular weight” without specifying
the meaning of that term. The parties agree that “molec-
ular weight” could refer to Mp, Mw, or Mn. And they agree
that each of these measures is calculated in a different
way and would typically yield a different result for a
given polymer sample. But the claim on its face offers no
guidance on which measure of “molecular weight” the
claims cover.
    There is no express definition of “molecular weight” in
the ’808 patent specification. Nowhere in the specification
are the terms Mp, Mw, or Mn used. Neither party argues
to the contrary. Instead, in its supplemental briefing to
this court, Teva argues that:
     The factual findings establish that to a skilled ar-
     tisan, ‘average molecular weight’ has a presumed
     meaning in this context, which means that the pa-
     tent’s specification resolves any ambiguity; that
     the presumed meaning is consistent with Figure 1
     of the specification; and that the only statement
     causing ambiguity in the prosecution history was
     an error. . . . And no intrinsic evidence defeats
     that definite meaning—not Figure 1, and not the
     prosecution history.
Appellees’ Supp. Br. 1; see also id. at 4–7 (Section 2.
Heading: “The District Court Did Not Clearly Err in
Finding That The Key Term Has A Presumed Meaning In
The Art”). Teva’s recitation of what was found below is
inaccurate. The district court did not find that the term
TEVA PHARMACEUTICALS USA   v. SANDOZ INC.               11


molecular weight or average molecular weight had a
presumed meaning in the art. And if there were such a
finding, it would not have been supported by the record in
this case. All parties agree that the term “molecular
weight” or “average molecular weight” in the Group I
claims could refer to any of the three weight measures Mp,
Mn, or Mw. Even Teva’s expert Dr. Grant repeatedly
admitted that the term molecular weight has no default
meaning to one of skill in the art. J.A. 3096-97. And
while it is true that the district court used the word
“presumed” once in its indefiniteness section, the use was
in characterizing Teva’s argument, not in making a fact
finding: “Thus, Teva (and Dr. Grant) conclude, Mp can be
read from the chromatogram generated by SEC without
any ‘further calculation’ and would be understood by a
person of ordinary skill in the art to be the presumed
meaning of [average molecular weight] in the context of
the patents-in-suit.” Markman Order at 588.
    The district court’s determination about how a skilled
artisan would understand the way in which SEC-
generated chromatogram data reflects molecular weight is
a question of fact. And we see no clear error in that fact
finding—that one of skill in the art could read Mp from a
chromatogram without further calculation and that Mw or
Mn would both require further calculations. We see no
clear error in the district court’s decision to credit Dr.
Grant’s testimony that Figure 1 was created by trans-
forming data from a chromatogram to the curves depicted
in Figure 1. Nor do we see clear error in its acceptance of
Dr. Grant’s opinion that a person of ordinary skill would
understand that the process of transforming such data
could cause the peaks of each curve to shift slightly such
that a person of skill would understand that the listed
molecular weights fall approximately at the peaks of the
curves, i.e., Mp. While Dr. Grant’s argument that the
peak positions on the curves are within a margin of error
12                TEVA PHARMACEUTICALS USA    v. SANDOZ INC.


(which he admits is as high as an error of 16.7%) is rela-
tively cursory and unexplained, see J.A. 1016–17, none-
theless we do not find the district court’s reliance on it
clearly erroneous. But accepting these fact findings does
not, as Teva suggests, mean that there now exists a
presumption regarding the meaning of the claim term in
the art in general or in the context of this patent.
    To the extent that Teva argues that the meaning of
“molecular weight” in the context of patents-in-suit is
itself a question of fact, it is wrong. See Teva, 135 S. Ct.
at 841–42. A party cannot transform into a factual mat-
ter the internal coherence and context assessment of the
patent simply by having an expert offer an opinion on it.
The internal coherence and context assessment of the
patent, and whether it conveys claim meaning with
reasonable certainty, are questions of law. The meaning
one of skill in the art would attribute to the term molecu-
lar weight in light of its use in the claims, the disclosure
in the specification, and the discussion of this term in the
prosecution history is a question of law. The district court
should not defer to Dr. Grant’s ultimate conclusion about
claim meaning in the context of this patent nor do we
defer to the district court on this legal question. To the
extent that Teva argues that this ultimate determination
deserves deference, it is in error. To the extent that Teva
or the dissent suggests that the specification’s disclosure
of SEC would “infer” that this claim term, molecular
weight, in this patent refers to Mp, such an inference is
part of the legal analysis, not a fact finding to be given
deference. Determining the meaning or significance to
ascribe to the legal writings which constitute the intrinsic
record is legal analysis. The Supreme Court made clear
that the factual components include “the background
science or the meaning of a term in the relevant art
during the relevant time period.” Id. at 841. Teva cannot
transform legal analysis about the meaning or signifi-
TEVA PHARMACEUTICALS USA   v. SANDOZ INC.                13


cance of the intrinsic evidence into a factual question
simply by having an expert testify on it. Id. at 841 (“ex-
perts may be examined to explain terms of art, and the
state of the art, at any given time, but they cannot be
used to prove the proper or legal construction of any
instrument of writing” (citation omitted)). Determining
the significance of disclosures in the specification or
prosecution history is also part of the legal analysis.
Understandings that lie outside the patent documents
about the meaning of terms to one of skill in the art or the
science or state of the knowledge of one of skill in the art
are factual issues. Even accepting as correct the district
court’s factual determinations about SEC and the transfer
of chromatogram data to create Figure 1, these facts do
not resolve the ambiguity in the Group I claim about the
intended molecular weight measure.
    To determine whether one of skill in the art would be
reasonably certain that the claim’s use of molecular
weight is Mp, we consider as well the prosecution history.
Statements made during prosecution history are relevant
to claim construction. See Teva, 135 S. Ct. at 841; Phil-
lips v. AWH Corp., 415 F.3d 1303, 1317 (Fed. Cir. 2005).
Applicants can define (lexicography), explain, or disavow
claim scope during prosecution.       And whether their
statements or disclaimers impact the meaning of a claim
term in a given patent is a legal question, not a factual
one. A statement made during prosecution of related
patents may be properly considered in construing a term
common to those patents, regardless of whether the
statement pre- or post-dates the issuance of the particular
patent at issue. Microsoft Corp. v. Multi-Tech Sys., Inc.,
357 F.3d 1340, 1350 (Fed. Cir. 2004). The parties do not
point to any portion of the ’808 patent’s prosecution
history that is relevant to the construction of “molecular
weight.” However, they point to, and the district court
considered, statements about the meaning of “molecular
14                TEVA PHARMACEUTICALS USA     v. SANDOZ INC.


weight” made during the prosecution of the ’847 and ’539
patents which are both continuations of the ’808 patent. 5
Such statements are legally relevant to the meaning one
of skill in the art would attribute to the identical term in
the ‘808 patent. In the prosecution of both patents, the
examiner rejected the claims as indefinite because the
term average molecular weight was meaningless without
specifying whether Mp, Mn, or Mw should be used. See
J.A. 3220 (“The term ‘average’ molecular weight . . . is
meaningless as a limitation without specifying its basis,
e.g. weight average molecular weight, number average
molecular weight, etc.”); J.A. 3245 (“[T]he term ‘average
molecular weight’ . . . is indefinite since its method of
measurement is not specified, i.e. number average molec-
ular weight, weight average molecular weight, average
molecular weight as determined by light scattering, etc.”).
The ’808, ’847, and ’539 patents share a nearly identical
specification, and all three patents identically include
Example 1 and Figure 1, discussed above. That these
applications containing the same Example 1 and Figure 1
as the ’808 patent were rejected for indefiniteness sug-
gests that, contrary to Teva’s position, the specification
does not conclusively establish that in the context of these
patents a person of ordinary skill in the art would con-
clude that the meaning of “molecular weight” is Mp.


     5  Prior to the expiration of the other patents-in-suit,
Teva did not dispute the relevance of the prosecution
history of related patents to the construction of the term
“molecular weight.” See Markman Order at 592; J.A.
1018. Now Teva argues that “the prosecution history of
later patents . . . cannot override the specification or
invalidate the patent.” Appellees’ Supp. Br. 1; see also id.
at 12. But we have said before, and reaffirm today, that
past and future prosecution of related patents may be
relevant to the construction of a given claim term.
TEVA PHARMACEUTICALS USA    v. SANDOZ INC.                15


     In response to the indefiniteness rejection concerning
the meaning of “molecular weight” during the prosecution
of the ’847 patent, the earlier of the two continuations, the
applicants argued that the term “molecular weight” was
not indefinite because “[o]ne of ordinary skill in the art
could understand that kilodalton units implies a weight
average molecular weight,” i.e., Mw. J.A. 3229. To be
clear, this was the only basis which the applicant argued
in response to the indefiniteness rejection. And the
applicant was successful. Defining “molecular weight” as
Mw, as the applicant did in response to the rejection, was
what overcame the rejection. The district court heard
testimony that the statement made during the prosecu-
tion of the ’847 patent was scientifically erroneous be-
cause each type of “molecular weight” can be expressed in
kilodaltons. Markman Order at 592. The fact finding by
the district court—that one of skill in the art would un-
derstand that each type of “molecular weight” could be
expressed in kilodaltons—is not clearly erroneous. How-
ever, the fact that Mw, Mn, and Mp can each be expressed
in kilodaltons does not erase the confusion created by the
patentee about its claim scope. Regardless of the scien-
tific accuracy of the statement, a person of ordinary skill
in the art would have understood that the applicants
defined the term “molecular weight” as Mw to gain allow-
ance of the claims. This is a legal conclusion unaffected
by the scientific error made during prosecution. To the
extent that the dissent claims that the significance to be
given to the patentee’s express definition of molecular
weight as Mw, made to overcome a rejection, is a question
of fact, the dissent is wrong. The determination of the
significance of statements made during prosecution to the
claim construction is a question of law.
   The examiner required the applicants to provide a
meaning for “molecular weight” and they provided one:
Mw. The fact that their explanation contained further
16                TEVA PHARMACEUTICALS USA   v. SANDOZ INC.


elaboration which itself included a scientific error does
not undermine the statement’s legal import. “The public
notice function of a patent and its prosecution history
requires that a patentee be held to what he declares
during the prosecution of his patent.” Springs Window
Fashions LP v. Novo Indus., L.P., 323 F.3d 989, 995 (Fed.
Cir. 2003). We have held patentees to statements con-
taining errors made during prosecution where, for exam-
ple, nothing in the statement was at odds with the plain
language of the claims or the specification. See id. at
995–96; see also Hockerson-Halberstadt, Inc. v. Avia Grp.
Int’l, Inc., 222 F.3d 951, 957 (Fed. Cir. 2000) (rejecting
patentee’s “request for a mulligan that would erase from
the prosecution history the inventor’s disavowal of a
particular aspect of a claim term’s meaning” despite
patentee’s argument that a person of ordinary skill would
have understood the statement during prosecution to be
erroneous); cf. Biotec Biologische Naturverpackungen
GmbH v. Biocorp, Inc., 249 F.3d 1341, 1348 (Fed. Cir.
2001) (declining to limit claims by erroneous statement
made during prosecution that was contrary to the plain
language of the claims, the specification, and other state-
ments made during prosecution). Given the role of the
statement in gaining allowance of the claims, a person of
ordinary skill in the art would have understood the appli-
cants to have defined “molecular weight” to mean Mw, and
the fact that any of the measures (Mw, Mn or Mp) can be
expressed in kilodaltons, does not change the significance
of the choice made by the patentee, Mw, to overcome the
rejection. And importantly, this determination is part of
the legal analysis, not as the dissent claims, one of the
fact findings to which we owe deference.
    During the prosecution of the ’539 patent, the appli-
cants responded to a nearly identical indefiniteness
rejection to the term “molecular weight” by arguing that a
person “of ordinary skill in the art, upon reviewing the
TEVA PHARMACEUTICALS USA    v. SANDOZ INC.                17


specification, would understand that ‘average molecular
weight’ refers to the molecular weight at the peak of the
molecular distribution curve in Figure 1,” i.e., Mp. J.A.
3258. Here too the specification was identical to the ’808
and ’847 patents in all respects relevant to the molecular
weight question, and the examiner found the specification
did not provide the reasonable certainty required for
definiteness. The patentee overcame that rejection by
again defining which measure of molecular weight to use,
in that case Mp.
     To summarize, it is undisputed that “molecular
weight” or average molecular weight can be ascertained
by any of three possible measures: Mp, Mn, and Mw. The
claims do not indicate which measure to use. The specifi-
cation never defines molecular weight or even mentions
Mp, Mw, or Mn. And the term “average molecular weight”
does not have a plain meaning to one of skill in the art.
The district court fact findings regarding how one of skill
in the art would understand the way in which a curve
created with chromatogram data reflects molecular
weights was not clearly erroneous. Its fact findings about
the additional calculations that would be required to
determine Mw or Mn are not clearly erroneous. Its fact
findings about how a skilled artisan would accept a curve
“shift” when converting chromatogram data to a curve
such as that illustrated in Figure 1 are not clearly errone-
ous. A skilled artisan, knowing a shift might occur, would
still not be reasonably certain in light of the entire record
as to which type of average was intended. During prose-
cution of the related ’847 and ’539 patents, which with
respect to molecular weight have identical specifications,
examiners twice rejected the term “molecular weight” as
indefinite for failing to disclose which measure of molecu-
lar weight to use (Mp, Mn, or Mw). And the patentee in
one instance stated that it was Mw and in the other stated
it was Mp. We find no clear error in the district court’s
18                TEVA PHARMACEUTICALS USA    v. SANDOZ INC.


fact finding that one of the statements contained a scien-
tifically erroneous claim. We hold that claim 1 is invalid
for indefiniteness by clear and convincing evidence be-
cause read in light of the specification and the prosecution
history, the patentee has failed to inform with reasonable
certainty those skilled in the art about the scope of the
invention. On this record, there is not reasonable certain-
ty that molecular weight should be measured using Mp.
This is the legal question—and on this question—we
reverse the district court.
     AFFIRMED-IN-PART, REVERSED-IN-PART
                          COSTS
     No costs.
  United States Court of Appeals
      for the Federal Circuit
                 ______________________

    TEVA PHARMACEUTICALS USA, INC.,
 TEVA PHARMACEUTICAL INDUSTRIES, LTD.,
 TEVA NEUROSCIENCE, INC., YEDA RESEARCH
       AND DEVELOPMENT CO., LTD.,
             Plaintiffs-Appellees

                            v.

 SANDOZ, INC., MOMENTA PHARMACEUTICALS
                    INC.,
             Defendants-Appellants

 MYLAN PHARMACEUTICALS INC., MYLAN INC.,
          NATCO PHARMA LTD.,
           Defendants-Appellants

         SANDOZ INTERNATIONAL GMBH,
                 NOVARTIS AG,
                    Defendants
              ______________________

      2012-1567, 2012-1568, 2012-1569, 2012-1570
               ______________________

MAYER, Circuit Judge, dissenting.
    “[I]n some instances, a factual finding may be close to
dispositive of the ultimate legal question of the proper
meaning of [a claim] term in the context of [a] patent.”
Teva Pharm. USA, Inc. v. Sandoz, Inc., 135 S. Ct. 831,
841–42 (2015). This is such a case. After carefully evalu-
ating the testimony of the parties’ experts, the district
2                  TEVA PHARMACEUTICALS USA    v. SANDOZ INC.



court made a factual finding that an artisan skilled in the
art of polypeptide synthesis would have inferred from the
use of an analytic technique known as size exclusion
chromatography (“SEC”) in U.S. Patent No. 5,800,808 (the
“’808 patent”) that the term “molecular weight” referred
to peak average molecular weight. Teva Pharm. USA,
Inc. v. Sandoz Inc., 810 F. Supp. 2d 578, 588 (S.D.N.Y.
2011) (“District Court Decision”). The court further found
that a person of ordinary skill in the art would have
accepted a statement made by the patentee during prose-
cution of U.S. Patent No. 6,939,539 (the “’539 patent”) as
“proof” of what was meant by the term “molecular
weight.” Id. at 592. * Because neither of these factual
findings is clearly erroneous, we are not free to disregard
or discount them in assessing whether the ’808 patent
withstands definiteness scrutiny. See Pullman-Standard
v. Swint, 456 U.S. 273, 287 (1982) (emphasizing that
Federal Rule of Civil Procedure 52(a) “does not make
exceptions or purport to exclude certain categories of
factual findings from the obligation of a court of appeals
to accept a district court’s findings unless clearly errone-
ous”). I therefore respectfully dissent.
                              I.
     In some cases, a trial court can decide an indefinite-
ness dispute based solely on the intrinsic evidence. When
a district court’s review is confined to the intrinsic record,
its conclusion on indefiniteness will be a legal determina-
tion which we can appropriately review de novo. See
Teva, 135 S. Ct. at 841. In many instances, however,


    *    Teva Pharmaceuticals USA, Inc. and related par-
ties (collectively “Teva”) own the ’808 patent, which is the
sole patent at issue in this appeal. The ’808 patent is
related to both the ’539 patent and U.S. Patent No.
6,620,847 (the “’847 patent”), and the three patents share
substantially identical specifications.
TEVA PHARMACEUTICALS USA     v. SANDOZ INC.                  3



particularly where complex technology is at issue, a trial
court will be required to look outside a patent and its
prosecution history in order to fully apprehend matters
such as “the background science or the meaning of a term
in the relevant art during the relevant time period.” Id.;
see Seymour v. Osborne, 78 U.S. 516, 546 (1871) (empha-
sizing that a patent may be “so interspersed with tech-
nical terms and terms of art that the testimony of
scientific witnesses is indispensable to a correct under-
standing of its meaning”). Those laboring in different
fields of scientific endeavor often speak with words drawn
from specialized lexicons, and in many cases it is only by
delving into the background science and thoroughly
evaluating the testimony of competing experts that a trial
court can make an informed determination as to whether
a claim provides a skilled artisan with reasonable certain-
ty as to the scope of an invention. See Loom Co. v. Hig-
gins, 105 U.S. 580, 585 (1881) (explaining that skilled
artisans “understand the language of their brother scien-
tist[s]”).
    The court here is once again led astray by its failure
to afford sufficient deference to the trial court’s findings of
fact. See Teva, 135 S. Ct. at 842. The district court
engaged in extensive fact-finding about the background
science reflected in the ’808 patent. See id. at 840 (“[T]his
case provides a perfect example of the factfinding that
sometimes underlies claim construction: The parties here
presented the District Court with competing fact-related
claims by different experts, and the District Court re-
solved the issues of fact that divided those experts.”).
After considering expert declarations and deposition
testimony—and holding two hearings—the court deter-
mined that the term “molecular weight” was not indefi-
nite because a skilled artisan would have understood its
meaning. See District Court Decision, 810 F. Supp. 2d at
587–95. Relying in significant measure on the testimony
of Dr. Gregory Grant, Teva’s expert, the trial court made
4                 TEVA PHARMACEUTICALS USA     v. SANDOZ INC.



three key factual determinations: (1) a person of ordinary
skill in the art of polypeptide synthesis would infer from
the use of the SEC method disclosed in the specification of
the ’808 patent that the term “molecular weight” referred
to peak average molecular weight, id. at 589–90; (2) a
skilled artisan would not rely upon a statement Teva
made when prosecuting the ’847 patent that the expres-
sion of molecular weight in kilodalton units “implie[d] a
weight average molecular weight,” because that state-
ment rested on obvious scientific error, id. at 591–92; and
(3) that artisan would instead rely on Teva’s affirmative
statement, made while prosecuting the ’539 patent, that
“molecular weight” meant peak average molecular weight,
id. at 592.
    We must be “mindful that we are a court of review,
not of first view,” Cutter v. Wilkinson, 544 U.S. 709, 718
n.7 (2005), and that our duty is to evaluate each case in
light of the facts as the trial court has found them. This
court’s conclusion that the ’808 patent is fatally indefinite
hinges on the fact that Teva made divergent statements
as to the meaning of “molecular weight” when prosecuting
the ’847 and ’539 patents. Ante at 17. That conclusion,
however, cannot be reconciled with the district court’s
express factual finding that a skilled artisan would not
rely on the statement Teva made as to the meaning of
“molecular weight” when prosecuting the ’847 patent
because it was scientifically incorrect. Peak average
molecular weight, weight average molecular weight, and
number average molecular weight are all expressed in
kilodaltons. See District Court Decision, 810 F. Supp. 2d
at 592. Accordingly, Teva’s statement that the use of
kilodalton units implied that “molecular weight” meant
weight average molecular weight was a non sequitur and,
as the district court correctly found, a skilled artisan
would not have relied upon it. See id.
    As this court has repeatedly made clear, obviously er-
roneous statements in the prosecution file carry little
TEVA PHARMACEUTICALS USA   v. SANDOZ INC.                 5



weight in determining claim meaning. See, e.g., Rambus
Inc. v. Infineon Techs. AG, 318 F.3d 1081, 1090 (Fed. Cir.
2003) (emphasizing that an “incorrect statement in the
prosecution history [did] not govern the meaning of the
claims”); Biotec Biologische Naturverpackungen GmbH v.
Biocorp, Inc., 249 F.3d 1341, 1348 (Fed. Cir. 2001) (“An
error in the prosecution record must be viewed as are
errors in documents in general; that is, would it have been
apparent to the interested reader that an error was made,
such that it would be unfair to enforce the error.”). Fur-
thermore, a single statement by Teva during prosecution
of the ’847 patent—made years after the ’808 patent
issued—should not be deemed dispositive on the question
of whether the ’808 patent is sufficiently definite. See
Athletic Alternatives, Inc. v. Prince Mfg., Inc., 73 F.3d
1573, 1580 (Fed. Cir. 1996) (emphasizing that the ambi-
guity of the prosecution history made it “unhelpful as an
interpretive resource” for determining the meaning of a
claim term). While the prosecution history of one patent
in a chain may be used to construe the same term in both
earlier and later issued related patents with the same
specification, see Microsoft Corp. v. Multi-Tech Sys., Inc.,
357 F.3d 1340, 1350 (Fed. Cir. 2004), the court is unable
to cite to a single case in which a statement made in
prosecuting a later related patent was deemed sufficient,
standing alone, to render an earlier issued patent indefi-
nite. The prosecution history of the ’847 patent cannot
trump the disclosure in the specification of the ’808 patent
which, by describing the use of the SEC method, indicates
to a skilled artisan that “molecular weight” means peak
average molecular weight. See Vederi, LLC v. Google,
Inc., 744 F.3d 1376, 1382 (Fed. Cir. 2014) (emphasizing
that “the specification is the single best guide to the
meaning of a claim term” and that “the prosecution histo-
ry often lacks the clarity of the specification” (citations
and internal quotation marks omitted)).
6                 TEVA PHARMACEUTICALS USA     v. SANDOZ INC.



    According to the court, although Teva’s statement
that the use of kilodalton units “implie[d] a weight aver-
age molecular weight” was scientifically incorrect, a
skilled artisan would nonetheless have understood that
the applicants defined the term “molecular weight” as
weight average molecular weight to gain allowance of the
claims. Ante at 16. This argument is unconvincing. Read
as a whole, Teva’s statement that the use of kilodalton
units “implie[d] a weight average molecular weight” is
nonsensical, and a skilled artisan would not rely upon any
part of it. This is particularly true given that Teva con-
firmed, when subsequently prosecuting the ’539 patent,
that—consistent with the use of the SEC method dis-
closed in the specification—the term “molecular weight”
meant peak average molecular weight. See Elbex Video,
Ltd. v. Sensormatic Elecs. Corp., 508 F.3d 1366, 1372–73
(Fed. Cir. 2007) (concluding that an earlier, incorrect
statement in the prosecution history did not override a
later, correct statement as to claim scope).
    In assessing obviousness, what the prior art teaches is
a question of fact. See Graham v. John Deere Co., 383
U.S. 1, 17 (1966); see also Lighting Ballast Control LLC v.
Philips Elecs. N. Am. Corp., 744 F.3d 1272, 1307 (Fed.
Cir. 2014) (en banc) (O’Malley, J., dissenting), judgment
vacated sub nom. Lighting Ballast Control LLC v. Univer-
sal Lighting Techs., Inc., 135 S. Ct. 1173 (2015) (“Im-
portantly, one of the key fact questions in an obviousness
inquiry is what a prior art reference teaches—often, what
is claimed and described in a previously issued patent.
And, all findings regarding the scope and content of the
prior art are subject to clear error review.” (citation omit-
ted)). In assessing indefiniteness, likewise, a trial court’s
determination, based on expert testimony, as to what a
skilled artisan would glean from subsequently issued
patents and their prosecution histories is a factual finding
which can be set aside only for clear error. See Bose Corp.
v. Consumers Union of U.S., Inc., 466 U.S. 485, 498 (1984)
TEVA PHARMACEUTICALS USA    v. SANDOZ INC.                 7



(“It surely does not stretch the language of [Rule 52(a)] to
characterize an inquiry into what a person knew at a
given point in time as a question of ‘fact.’” (footnote omit-
ted)). We cannot substitute our assessment of the testi-
mony for that of the trial court simply because from our
appellate perch we might assess that testimony different-
ly. If we credit—as we must because it is not clearly
erroneous—the district court’s explicit finding that a
skilled artisan would not rely on the facially incorrect
statement made during prosecution of the ’847 patent,
there is no reasonable basis for concluding that the prose-
cution history of patents related to the ’808 patent would
create, in the mind of the skilled artisan, ambiguity
regarding the meaning of the term “molecular weight.”
    The court’s approach here contravenes binding prece-
dent. In Enzo Biochem Inc. v. Applera Corp., we held that
under Teva a trial court’s conclusion, based on expert
testimony, as to whether an example in the specification
disclosed “direct detection” was a “factual finding” which
was subject to clear error review. 780 F.3d 1149, 1156
(Fed. Cir. 2015). Likewise, in EON Corp. IP Holdings
LLC v. AT&T Mobility LLC, we held that a district court’s
determination, based on the testimony from experts, that
claims disclosed “complicated, customized computer soft-
ware” was a “factual finding[].” 785 F.3d 616, 624 (Fed.
Cir. 2015). Here, however, the court insists that the
determination, based on extensive expert testimony, that
a skilled artisan would not rely on a facially incorrect
statement made during prosecution of the ’847 patent was
“part of the legal analysis, not as the dissent claims, one
of the fact findings to which we owe deference.” Ante at
16. The court’s view that the universe of factual findings
to    which     we    owe     deference    includes    only
“[u]nderstandings that lie outside the patent documents,”
ante at 13, simply cannot be squared with Enzo and EON.
   Although the ultimate conclusion of indefiniteness
under 35 U.S.C. § 112 is a legal question, see Eidos Dis-
8                 TEVA PHARMACEUTICALS USA    v. SANDOZ INC.



play, LLC v. AU Optronics Corp., 779 F.3d 1360, 1364-65
(Fed. Cir. 2015), Teva mandates that the trial court’s
factual findings are to be respected, barring clear error,
and that the required legal analysis must be performed in
view of those findings, 135 S. Ct. at 841. Here, however,
the court takes the opposite tack, first embarking on an
independent review of the record and then considering, as
an afterthought, the important and carefully considered
factual findings made by the trial court.
                            II.
    In Nautilus, Inc. v. Biosig Instruments, Inc., the Su-
preme Court rejected this court’s view that a claim met
definiteness requirements so long as it was “amenable to
construction,” and, as construed, was not “insolubly
ambiguous.” 134 S. Ct. 2120, 2124 (2014) (citations and
internal quotation marks omitted). Because the district
court here relied on the now discarded “insolubly ambigu-
ous” standard when it held that the ’808 patent was not
invalid for indefiniteness, see District Court Decision, 810
F. Supp. 2d at 582, this case should be remanded so that
the court can take additional evidence as it deems appro-
priate and assess in the first instance whether the ’808
patent meets the more stringent Nautilus definiteness
standard. See 134 S. Ct. at 2124 (explaining “that a
patent is invalid for indefiniteness if its claims, read in
light of the specification delineating the patent, and the
prosecution history, fail to inform, with reasonable cer-
tainty, those skilled in the art about the scope of the
invention”).
    The ’808 patent will expire in September 2015. This
looming expiration date does not, however, permit us to
overstep our appellate role or feign first-hand experience
with the testimony or the technology. We are neither
equipped nor authorized to make the predicate factual
determinations necessary to assess whether the ’808
patent withstands definiteness scrutiny under the Nauti-
TEVA PHARMACEUTICALS USA   v. SANDOZ INC.              9



lus standard. See Anderson v. Bessemer City, 470 U.S.
564, 574 (1985) (emphasizing that “[t]he trial judge’s
major role is the determination of fact, and with experi-
ence in fulfilling that role comes expertise”).
