                 FOR PUBLICATION

  UNITED STATES COURT OF APPEALS
       FOR THE NINTH CIRCUIT


LINDA MESSICK,                            No. 13-15433
                  Plaintiff-Appellant,
                                             D.C. No.
                 v.                      3:12-cv-00693-SI

NOVARTIS PHARMACEUTICALS
CORPORATION,                                OPINION
             Defendant-Appellee.


     Appeal from the United States District Court
        for the Northern District of California
     Susan Illston, Senior District Judge, Presiding

                Argued and Submitted
        February 3, 2014—Seattle, Washington

                   Filed April 4, 2014

    Before: Raymond C. Fisher, Ronald M. Gould,
        and Morgan Christen, Circuit Judges.

                 Opinion by Judge Gould
2     MESSICK V. NOVARTIS PHARMACEUTICALS CORP.

                           SUMMARY*


                        Expert Testimony

     The panel reversed the district court’s summary judgment
in favor of Novartis Pharmaceuticals Corporation because the
district court improperly excluded expert testimony.

    Plaintiff offered Dr. Richard Jackson’s testimony on
osteonecrosis of the jaw and bisphosphonate-related
osteonecrosis of the jaw generally (“BRONJ”), and on the
causal link between plaintiff’s bisphosphonate treatment and
later development of BRONJ.

    The panel held that the district court erred by excluding
the causation testimony offered by the plaintiff’s expert, Dr.
Jackson, when it found the testimony to be irrelevant and
unreliable. The panel held that Dr. Jackson’s testimony
indicated that plaintiff’s bisphosphonate         use was a
substantial factor in her development of BRONJ, so the
testimony was relevant. The panel also held that the
testimony was reliable where Dr. Jackson testified a medical
condition was a substantial causative factor. The panel
concluded that Dr. Jackson’s expert testimony created a
genuine issue of material fact regarding the specific causal
link between plaintiff’s bisphosphonate treatment and her
development of osteonecrosis of the jaw.




  *
    This summary constitutes no part of the opinion of the court. It has
been prepared by court staff for the convenience of the reader.
     MESSICK V. NOVARTIS PHARMACEUTICALS CORP.               3

                         COUNSEL

Martin N. Buchanan (argued), Law Offices of Martin N.
Buchanan, San Diego, California; John A. Girardi, Girardi
Keese, Los Angeles, California; John J. Vecchione, Valad &
Vecchione, PC, Fairfax, Virginia, for Plaintiff-Appellant.

Frank Leone (argued), Katharine R. Latimer, Rebecca A.
Womeldorf, Hollingsworth LLP, Washington, D.C., for
Defendant-Appellee.


                         OPINION

GOULD, Circuit Judge:

    We consider when a district court should exclude expert
testimony at trial. Linda Messick contends that the district
court erred by excluding the causation testimony offered by
her expert, Dr. Richard Jackson, when it found the testimony
to be irrelevant and unreliable. Examining the district court’s
exclusion of expert testimony for an abuse of discretion, we
conclude that Dr. Jackson’s testimony was improperly
excluded, and we reverse and remand.

                               I

    In August 2000, Linda Messick was diagnosed with
breast cancer. In response to her development of osteoporosis
after chemotherapy and steroid therapy, Messick was treated
with the drug Zometa from May 7, 2002 to November 14,
2002. Zometa is a bisphosphonate, a class of drug commonly
used to treat multiple myeloma. Such drugs are used to
reduce or eliminate the possibility of skeletal-related
4    MESSICK V. NOVARTIS PHARMACEUTICALS CORP.

degeneration and injuries to which cancer patients are
particularly susceptible.     Novartis Pharmaceuticals
Corporation (“Novartis”) produces Zometa, which was
approved by the FDA in 2001 and 2002 for treating
hypercalcemia of malignancy and other conditions.

    After Messick encountered several dental problems, two
oral specialists examined her in November 2005 and
discovered osteonecrosis near three of her teeth. Both doctors
treated her under the assumption that she was suffering from
bisphosphonate-related osteonecrosis of the jaw (“BRONJ”),
a condition recognized by the American Association of Oral
and Maxillofacial Surgeons (“AAOMS”). According to the
AAOMS, a BRONJ diagnosis is appropriate for patients with
(1) current or previous treatment with bisphosphonates,
(2) exposed bone in the maxillofacial region that has lasted
for more than eight weeks, and (3) no history of radiation
therapy to the jaw. Other potential causes of osteonecrosis of
the jaw (“ONJ”) may include: peridontal and dental disease,
osteomyelitis, corticosteroid use, cancer, radiation therapy,
compromised immunity, and trauma. While ONJ may be
caused by many factors, the AAOMS’s diagnostic definition
of BRONJ sets out its unique features: it lasts more than eight
weeks and is not related to radiation therapy. Messick’s
BRONJ healed between March and October 2008, about three
years after diagnosis. She and her husband brought suit
against Novartis for strict products liability, negligent
manufacture, negligent failure to warn, breach of express and
implied warranty, and loss of consortium.

    To support her claims, Messick offered Dr. Jackson’s
testimony on ONJ and BRONJ generally, and on the causal
link between her bisphosphonate treatment and later
development of BRONJ. Dr. Jackson graduated from the
     MESSICK V. NOVARTIS PHARMACEUTICALS CORP.              5

University of the Pacific’s School of Dentistry in 1974,
completed his oral and maxillofacial surgery residency in
1982, and became Board Certified in oral maxillofacial
surgery in 1984. He belongs to several dental and oral
maxillofacial professional groups, and has published and
lectured on a variety of topics over many years. Dr. Jackson
has extensive experience diagnosing and treating ONJ,
including ONJ in patients who had been treated with
bisphosphonates, and he is the primary oral and maxillofacial
surgeon managing ONJ in the Sacramento area.

    While the action was pending in Multi-District Litigation
proceedings, Novartis filed a motion to exclude the specific
causation testimony of Messick’s experts and a motion
seeking summary judgment. After the case was remanded
and transferred, the district court granted those motions, and
Messick appeals the exclusion of Dr. Jackson’s causation
testimony and the ensuing grant of summary judgment to
Novartis.

                              II

    We review a ruling on the admissibility of expert
testimony for an abuse of discretion, Primiano v. Cook,
598 F.3d 558, 563 (9th Cir. 2010), but review de novo the
“construction or interpretation of . . . the Federal Rules of
Evidence, including whether particular evidence falls within
the scope of a given rule.” United States v. Durham,
464 F.3d 976, 981 (9th Cir. 2006) (citations omitted).

   The substantive issue here arises under the Federal Rules
of Evidence and California state products liability law.
Federal Rule of Evidence 702 governs expert testimony:
6    MESSICK V. NOVARTIS PHARMACEUTICALS CORP.

        A witness who is qualified as an expert by
        knowledge, skill, experience, training, or
        education may testify in the form of an
        opinion or otherwise if:

        (a) the expert’s scientific, technical, or other
        specialized knowledge will help the trier of
        fact to understand the evidence or to
        determine a fact in issue;

        (b) the testimony is based on sufficient facts
        or data;

        (c) the testimony is the product of reliable
        principles and methods; and

        (d) the expert has reliably applied the
        principles and methods to the facts of the
        case.

Fed. R. Evid. 702. Although Rule 702 should be applied with
a “liberal thrust” favoring admission, Daubert v. Merrell Dow
Pharm., Inc., 509 U.S. 579, 588 (1993), it requires that
“[e]xpert testimony . . . be both relevant and reliable,” Estate
of Barabin v. AstenJohnson, Inc., 740 F.3d 457, 463 (9th Cir.
2014) (alteration and omission in original) (quoting United
States v. Vallejo, 237 F.3d 1008, 1019 (9th Cir. 2001)).

    The relevancy bar is low, demanding only that the
evidence “logically advances a material aspect of the
proposing party’s case.” Daubert v. Merrell Dow Pharm.,
Inc., 43 F.3d 1311, 1315 (9th Cir. 1995) (“Daubert II”).
Relevancy depends on the particular law at issue because
“[e]xpert opinion testimony is relevant if the knowledge
     MESSICK V. NOVARTIS PHARMACEUTICALS CORP.                7

underlying it has a valid connection to the pertinent inquiry.”
Primiano, 598 F.3d at 565. Here, California state products
liability law requires only that a plaintiff show that the
defendant’s conduct was “more likely than not” a substantial
factor in causing the injury in order to prove specific
causation. See Saelzler v. Advanced Grp. 400, 23 P.3d 1143,
1152 (Cal. 2001).

    The district court excluded Dr. Jackson’s testimony as
irrelevant because of its view that his “differential diagnosis
only determines that Ms. Messick’s ONJ is related to her
bisphosphonate use, and he admits that a diagnosis of BRONJ
does not mean that bisphosphonates caused her ONJ.” The
district court applied too high a relevancy bar without
reference to California law. While Dr. Jackson never
explicitly stated that Messick’s bisphosphonate use caused
her BRONJ, he did say it was at least a substantial factor in
her development of BRONJ. Specifically, Dr. Jackson
equated Messick’s use of bisphosphonates leading to BRONJ
with the oxygen necessary to start a fire. Although one might
imagine more targeted testimony, Dr. Jackson’s testimony
indicates that Messick’s bisphosphonate use was a substantial
factor in her development of BRONJ, so his testimony is
relevant.

    The district court also abused its discretion in finding Dr.
Jackson’s testimony to be unreliable. The reliability
threshold requires that the expert’s testimony have “a reliable
basis in the knowledge and experience of the relevant
discipline.” Kumho Tire Co., Ltd. v. Carmichael, 526 U.S.
137, 149 (1999) (citation and alterations omitted). The court
must determine “whether the reasoning or methodology
underlying the testimony is scientifically valid.” Daubert,
509 U.S. at 592–93.
8    MESSICK V. NOVARTIS PHARMACEUTICALS CORP.

    We recently acknowledged the district court’s duty to “act
as a ‘gatekeeper’ to exclude junk science that does not meet
Federal Rule of Evidence 702’s reliability standards.”
Barabin, 740 F.3d at 463 (quoting Ellis v. Costco Wholesale
Corp., 657 F.3d 970, 982 (9th Cir. 2011)). To aid courts in
exercising this gatekeeping role, the Supreme Court has
suggested a non-exclusive and flexible list of factors that a
court may consider when determining the reliability of expert
testimony, including: (1) whether a theory or technique can
be tested; (2) whether it has been subjected to peer review
and publication; (3) the known or potential error rate of the
theory or technique; and (4) whether the theory or technique
enjoys general acceptance within the relevant scientific
community. Daubert, 509 U.S. at 592–94.

    We and our sister circuits have held that a reliable
differential diagnosis may form the basis of an expert’s
causation testimony. See Clausen v. M/V NEW CARISSA,
339 F.3d 1049, 1057 (9th Cir. 2003); see also Ervin v.
Johnson & Johnson, Inc., 492 F.3d 901, 904 (7th Cir. 2007)
(noting that “[a] differential diagnosis satisfies a Daubert
analysis if the expert uses reliable methods”); McClain v.
Metabolife Int’l, Inc., 401 F.3d 1233, 1252 (11th Cir. 2005)
(detailing a reliable differential diagnostic process); Goebel
v. Denver & Rio Grande W. R.R. Co., 346 F.3d 987, 999
(10th Cir. 2003); Cooper v. Smith & Nephew, Inc., 259 F.3d
194, 202 (4th Cir. 2001) (holding that an expert’s opinion
based on a differential diagnosis is generally admissible);
Hardyman v. Norfolk & W. Ry. Co., 243 F.3d 255, 260 (6th
Cir. 2001) (“Differential diagnosis . . . is a standard scientific
technique of identifying the cause of a medical problem”);
Turner v. Iowa Fire Equip. Co., 229 F.3d 1202, 1208 (8th
Cir. 2000) (“[A] medical opinion about causation, based upon
a proper differential diagnosis, is sufficiently reliable to
      MESSICK V. NOVARTIS PHARMACEUTICALS CORP.                        9

satisfy Daubert.”); Baker v. Dalkon Shield Claimaints Trust,
156 F.3d 248, 253 (1st Cir. 1998) (stating that “‘differential
diagnosis’ is a standard medical technique”); Ambrosini v.
Labarraque, 101 F.3d 129, 140 (D.C. Cir. 1996) (describing
the appropriate use of differential diagnosis to prove specific
causation); McCullock v. H.B. Fuller Co., 61 F.3d 1038,
1043–44 (2d Cir. 1995); In re Paoli R.R. Yard PCD Litig.,
916 F.2d 829, 862 (3d Cir. 1990).1

    When an expert rules out a potential cause in the course
of a differential diagnosis, the “expert must provide reasons
for rejecting alternative hypotheses using scientific methods
and procedures and the elimination of those hypotheses must
be founded on more than subjective beliefs or unsupported
speculation.” Clausen, 339 F.3d at 1058 (internal quotation
marks and citation omitted); Daubert II, 43 F.3d at 1318
(requiring explanations based on “scientifically valid
principles”). The district court abused its discretion in
excluding Dr. Jackson’s opinion, which stated that a patient
without cancer or exposure to radiation in the mouth area
would not develop ONJ lasting for years without IV
bisphosphonate treatments. It excluded the testimony
because Dr. Jackson “never explained the scientific basis for
this conclusion.” But Dr. Jackson repeatedly referred to his
own extensive clinical experience as the basis for his
differential diagnosis, as well as his examination of Messick’s
records, treatment, and history. Medicine partakes of art as
well as science, and there is nothing wrong with a doctor
relying on extensive clinical experience when making a


  1
    The Fifth Circuit is alone in not yet explicitly approving the use of
differential diagnosis, although it suggested in a recent case that a
differential diagnosis may be admissible to prove specific causation. See
Johnson v. Arkema, Inc., 685 F.3d 452, 467–68 (5th Cir. 2012).
10   MESSICK V. NOVARTIS PHARMACEUTICALS CORP.

differential diagnosis. Dr. Jackson also relied on the
AAOMS definition of BRONJ in reaching his diagnosis and
causation conclusions, which itself persuasively uses the three
elements to distinguish BRONJ from other ONJ or delayed
healing conditions. These sources form an appropriate
scientific basis for his opinions, and the district court abused
its discretion in concluding otherwise. See Primiano,
598 F.3d at 567 (admitting expert’s testimony with “sufficient
basis in education and experience”).

    Although experts must provide scientifically sound
reasons for excluding potential causes, we have consistently
recognized the difficulties in establishing certainty in the
medical sciences. Id. at 565 (stating that “medical knowledge
is often uncertain” (quoting United States v. Sandoval-
Mendoza, 472 F.3d 645, 655 (9th Cir. 2006))). The Supreme
Court has similarly stressed that “it would be unreasonable to
conclude that the subject of scientific testimony must be
‘known’ to a certainty; arguably, there are no certainties in
science.” Daubert, 509 U.S. at 590.

    Given the difficulties in establishing a medical cause and
effect relationship, “[c]ausation can be proved even when we
don’t know precisely how the damage occurred, if there is
sufficiently compelling proof that the agent must have caused
the damage somehow.” Kennedy v. Collagen Corp., 161 F.3d
1226, 1230 (9th Cir. 1998) (quoting Daubert II, 43 F.3d at
1314). Viewed in this context, it is perhaps unsurprising that
the AAOMS stated in a 2009 position paper that “the current
level of evidence does not fully support a cause-and-effect
relationship between bisphosphonate exposure and necrosis
of the jaws.” But the paper goes on to explain that while
“causality might never be proven, emerging experimental and
epidemiologic studies have established a firm foundation for
     MESSICK V. NOVARTIS PHARMACEUTICALS CORP.                 11

a strong association between monthly IV bisphosphonate
therapy and the development of BRONJ.”

    Because of that inherent uncertainty, we do not require
that an expert be able to identify the sole cause of a medical
condition in order for his or her testimony to be reliable. It is
enough that a medical condition be a substantial causative
factor. We hold that the district court abused its discretion in
excluding Dr. Jackson’s causation testimony when it found
that testimony to be unreliable largely because Dr. Jackson
could not “determine in a patient who has multiple risk
factors at one time which of those particular risk factors is
causing [the ONJ].” Such an unduly exacting standard goes
beyond the district court’s proper gatekeeping role.

     Dr. Jackson’s testimony should be admitted as relevant
and reliable. Remaining issues regarding the correctness of
his opinion, as opposed to its relevancy and reliability, are a
matter of weight, not admissibility. Kennedy, 161 F.3d at
1230–31. While the district court must act as a gatekeeper to
exclude “junk science” under Daubert, Federal Rule of
Evidence 702(a) includes within its scope all evidence that
would “help the trier of fact . . . to determine a fact in issue.”
A doctor using a differential diagnosis grounded in significant
clinical experience and examination of medical records and
literature can certainly aid the trier of fact and cannot be
considered to be offering “junk science.”

                               III

    Summary judgment is proper when the moving party
shows “there is no genuine dispute as to any material fact and
the movant is entitled to judgment as a matter of law.” Fed.
R. Civ. P. 56(a). A court must view the evidence in the light
12   MESSICK V. NOVARTIS PHARMACEUTICALS CORP.

most favorable to the non-moving party, Scott v. Harris,
550 U.S. 372, 378 (2007), and “there must be evidence on
which the jury could reasonably find for the plaintiff,”
Anderson v. Liberty Lobby, Inc., 477 U.S. 242, 252 (1986).
We review the district court’s grant of summary judgement
de novo. Lust ex rel. Lust v. Merrell Dow Pharm., Inc.,
89 F.3d 594, 596–97 (9th Cir. 1996).

    Here, the district court based its grant of summary
judgment entirely on the fact that Messick had no expert
causation testimony once all of her experts had been
excluded. The admissibility, correctly viewed, of Dr.
Jackson’s expert testimony creates a genuine issue of material
fact regarding the specific causal link between Messick’s
bisphosphonates treatment and her development of ONJ. We
reverse the district court’s grant of summary judgment.

     REVERSED AND REMANDED.
