NOTICE: This opinion is subject to formal revision before publication in the preliminary print of the United States Reports. Readers are requested to notify the Reporter of Decisions, Supreme Court of the United States, Washington, D. C. 20543, of any typographical or other formal errors, in order that corrections may be made before the preliminary print goes to press.
SUPREME COURT OF THE UNITED STATES
MINERVA SURGICAL, INC., PETITIONER v.
HOLOGIC, INC., et al.
on writ of certiorari to the united states court of appeals for the federal circuit
[June 29, 2021]
Justice Kagan delivered the opinion of the Court.
In Westinghouse Elec. & Mfg. Co.
v. Formica Insulation Co.
266 U.S. 342
, 349 (1924), this Court approved the “well settled” patent-law doctrine of “assignor estoppel.” That doctrine, rooted in an idea of fair dealing, limits an inventor’s ability to assign a patent to another for value and later contend in litigation that the patent is invalid. The question presented here is whether to discard this century-old form of estoppel. Continuing to see value in the doctrine, we decline to do so. But in upholding assignor estoppel, we clarify that it reaches only so far as the equitable principle long understood to lie at its core. The doctrine applies when, but only when, the assignor’s
claim of invalidity contradicts explicit or implicit representations he made in assigning the patent.
Inventors look to the patent system to obtain valuable rights. A typical patent application, filed with the U. S. Patent and Trademark Office (PTO), includes a written description and drawing of the invention and one or more claims particularly setting out the invention’s scope. See 35 U. S. C. §§111–113. The application also usually contains an inventor’s oath—a statement attesting that the applicant is “the original inventor” of the “claimed invention,” so that he is entitled to the patent sought. §115. If the PTO decides that the invention meets the “conditions for patentability”—mainly, that the invention is useful, novel, and non-obvious—it will issue a patent to the inventor. See §§101–103. That award gives the inventor the right to exclude others from making, using, or selling the invention until the patent expires (currently, 20 years after the application date). See §154.
The invention sparking this lawsuit is a device to treat abnormal uterine bleeding, a medical condition affecting many millions of women. Csaba Truckai, a founder of the company Novacept, Inc., invented the device—called the NovaSure System—in the late 1990s. He soon afterward filed a patent application, and assigned his interest in the application—as well as in any future “continuation applications”—to Novacept.[1
] The NovaSure System, as described in Truckai’s patent application, uses an applicator head to destroy targeted cells in the uterine lining. To avoid unintended burning or ablation (tissue removal), the head is “moisture permeable,” meaning that it conducts fluid out of the uterine cavity during treatment. The PTO issued a patent, and in 2001 the Food and Drug Administration (FDA) approved the device for commercial distribution. But neither Truckai nor Novacept currently benefits from the NovaSure System patent. In 2004, Novacept sold its assets, including its portfolio of patents and patent applications, to another company (netting Truckai individually about $8 million). And in another sale, in 2007, respondent Hologic, Inc. acquired all patent rights in the NovaSure System. Today, Hologic sells that device throughout the United States.
Not through with inventing, Truckai founded in 2008 petitioner Minerva Surgical, Inc. There, he developed a supposedly improved device to treat abnormal uterine bleeding. Called the Minerva Endometrial Ablation System, the device (like the NovaSure System) uses an applicator head to remove cells in the uterine lining. But the new device, relying on a different way to avoid unwanted ablation, is “moisture impermeable”: It does not remove any fluid during treatment. The PTO issued a patent for the device, and in 2015 the FDA approved it for commercial sale.
Meanwhile, in 2013, Hologic filed a continuation application requesting to add claims to its patent for the NovaSure System. Aware of Truckai’s activities, Hologic drafted one of those claims to encompass applicator heads generally, without regard to whether they are moisture permeable. The PTO in 2015 issued the altered patent as requested.
A few months later, Hologic sued Minerva for patent infringement. Minerva rejoined that its device does not infringe. But more relevant here, it also asserted that Hologic’s amended patent is invalid. The essential problem, according to Minerva, is that the new, broad claim about applicator heads does not match the invention’s description, which addresses their water-permeability. See Defendant Minerva’s Opening Brief in Support of Its Motion for Partial Summary Judgment in No. 15–cv–1031 (Del.), Doc. 300, pp. 8–9, 13–15; see also Festo Corp.
v. Shoketsu Kinzoku Kogyo Kabushiki Co.
535 U.S. 722
, 736 (2002) (“What is claimed by the patent application must be the same” as what is described). In response, Hologic invoked the doctrine of assignor estoppel. Because Truckai assigned the original patent application, Hologic argued, he and Minerva (essentially, his alter-ego) could not impeach the patent’s validity. The District Court agreed that assignor estoppel barred Minerva’s invalidity defense, and also ruled that Minerva had infringed Hologic’s patent. See 325 F. Supp. 3d 507, 524–525, 532 (Del. 2018). At a trial on damages, a jury awarded Hologic about $5 million.
The Court of Appeals for the Federal Circuit mainly upheld the judgment, focusing on assignor estoppel. The court first “decline[d] Minerva’s invitation to ‘abandon [that] doctrine.’ ” 957 F.3d 1256, 1267 (2020). Citing both this Court’s precedents and equitable principles, the court affirmed the doctrine’s “continued vitality.” Id.,
at 1268. An assignor, the court stated, “should not be permitted to sell something and later to assert that what was sold is worthless, all to the detriment of the assignee.” Id.
at 1265. The assignor makes an “implicit representation” that the rights “he is assigning (presumably for value) are not worthless.” Ibid.
It would “work an injustice,” the court reasoned, to “allow the assignor to make that representation at the time of assignment (to his advantage) and later to repudiate it (again to his advantage).” Ibid.
(quoting Diamond Scientific Co.
v. Ambico, Inc.
, 848 F.2d 1220, 1224 (CA Fed. 1988)). The court then applied assignor estoppel to bar Truckai and Minerva from raising an invalidity defense. Here, the court rejected Minerva’s argument that because “Hologic broadened the claims” after “Truckai’s assignment,” it would “be unfair to block Truckai (or Minerva) from challenging the breadth of those claims.” 957 F. 3d, at 1268. Relying on circuit precedent, the court deemed it “irrelevant that, at the time of the assignment, the inventor’s patent application[ ] w[as] still pending” and that the assignee “may have later amended the claims” without the inventor’s input. Ibid.
(quoting Diamond Scientific
, 848 F. 2d, at 1226).
We granted certiorari, 592 U. S. ___ (2021), to consider the important issues raised in the Federal Circuit’s judgment. Assignor estoppel, we now hold, is well grounded in centuries-old fairness principles, and the Federal Circuit was right to uphold it. But the court failed to recognize the doctrine’s proper limits. The equitable basis of assignor estoppel defines its scope: The doctrine applies only when an inventor says one thing (explicitly or implicitly) in assigning a patent and the opposite in litigating against the patent’s owner.
Courts have long applied the doctrine of assignor estoppel to deal with inconsistent representations about a patent’s validity. The classic case (different in certain respects from the one here) begins with an inventor who both applies for and obtains a patent, then assigns it to a company for value. Later, the inventor/assignor joins a competitor business, where he develops a similar—and possibly infringing—product. When the assignee company sues for infringement, the assignor tries to argue—contrary to the (explicit or implicit) assurance given in assigning the patent—that the invention was never patentable, so the patent was
never valid. That kind of about-face is what assignor estoppel operates to prevent—or, in legalese, estop. As one of the early American courts to use the doctrine held: The assignor is not “at liberty to urge [invalidity] in a suit upon his own patent against a party who derives title to that patent through him.” Woodward
v. Boston Lasting Mach. Co.
, 60 F. 283, 284 (CA1 1894). Or as the Federal Circuit held in modern times: The assignor’s explicit or “implicit representation” that the patent he is assigning is “not worth- less . . . deprive[s] him of the ability to challenge later the [patent’s] validity.” Diamond Scientific
848 F. 2d, at 1224.
Assignor estoppel got its start in late 18th-century England and crossed the Atlantic about a hundred years later. In the first recorded case, Lord Kenyon found that a patent assignor “was by his own oath and deed estopped” in an infringement suit from “attempt[ing] to deny his having had any title to convey.” Oldham
(1789), as described in J. Davies, Collection of the Most Important Cases Respecting Patents of Invention and the Rights of Patentees 442 (1816); see Hayne
, 3 T. R. 439, 441, 100 Eng. Rep. 665, 666 (K. B. 1789) (recognizing the Oldham
holding). That rule took inspiration from an earlier doctrine—estoppel by deed—applied in real property law to prevent a conveyor of land from later asserting that he had lacked good title at the time of sale. See 2 E. Coke, The First Part of the Institutes of Laws of England 352a (Hargrave & Butler eds., 19th ed. 1832) (1628). Lord Kenyon’s new patent formulation of the doctrine grew in favor throughout the 1800s as an aspect of fair dealing: When “the Defendant sold and assigned th[e] patent to the Plaintiffs as a valid one,” it “does not lie in his mouth to say that the patent is not good.” Chambers
, 33 Beav. 374, 376, 55 Eng. Rep. 412 (1864); see Walton
, 8 C. B. N. S. 162, 187, 141 Eng. Rep. 1127, 1137 (C. P. 1860) (“The defendant, who has received a large sum for the sale of this patent, ought not to be allowed to raise any question as to its validity”). The earliest American decision applying the doctrine dates from 1880. See Faulks
, 3 F. 898 (CC SDNY). Within a decade or two, the doctrine was “so well established and generally accepted that citation of authority is useless.” Griffith
, 89 F. 313, 315 (CC SD Iowa 1893); see 2 W. Robinson, Law of Patents for Useful Inventions §787 (1890) (collecting cases).
This Court first considered—and unanimously approved—assignor estoppel in 1924, in Westinghouse
. Speaking through Chief Justice Taft, the Court initially invoked the doctrine’s uniform acceptance in the lower courts. The first decision applying assignor estoppel, the Court recounted, was soon “followed by a myriad.” 266 U. S., at 349. “[L]ater cases in nearly all the Circuit Courts of Appeal” were “to the same point” as the first, adding up to a full “forty-five years of judicial consideration and conclusion.” Ibid.
Such a “well settled” rule, in the Court’s view, should
“not [be] lightly disturb[ed].” Ibid.
And so it was not disturbed, lightly or otherwise. Rather, the Court added its own voice to that pre-existing “myriad.” We announced that an assignor “is estopped to attack” the “validity of a patented invention which he has assigned.” Ibid.
“As to the rest of the world,” the Court explained, “the patent may have no efficacy”; but “the assignor can not be heard to question” the assignee’s rights in what was conveyed. Ibid.
, like its precursor decisions, grounded assignor estoppel in a principle of fairness. “If one lawfully conveys to another a patented right,” the Court reasoned, “fair dealing should prevent him from derogating from the title he has assigned.” Id.,
at 350. After all, the “grantor purports to convey the right to exclude others”; how can he later say, given that representation, that the grantee in fact possesses no such right? Ibid.
The Court supported that view of equity by referring to estoppel by deed. See supra,
at 6. Under that doctrine, the Court explained, “a grantor of a deed of land” cannot “impeach[ ] the effect of his solemn act” by later claiming that the grantee’s title is no good. Westinghouse
266 U. S., at 350. “The analogy” was “clear”: There was “no reason why the principles of estoppel by deed should not apply to [the] assignment of a patent right.” Id.,
at 348, 350. In the latter context too, the Court held, the assignor could not fairly “attack” the validity of a right he had formerly sold. Id.,
After thus endorsing assignor estoppel, the Court made clear that the doctrine has limits. Although the assignor cannot assert in an infringement suit that the patent is invalid, the Court held that he can argue about how to construe the patent’s claims. Here, the Court addressed the role in patent suits of prior art—the set of earlier inventions (and other information) used to decide whether the specified invention is novel and non-obvious enough to merit a patent. Id.,
at 350. “Of course,” the Court said, the assignor cannot use prior art in an infringement suit “to destroy the patent,” because he “is estopped to do this.” Id.,
at 351. But he can use prior art to support a narrow claim construction—to “construe and narrow the claims of the patent, conceding their validity.” Id.,
at 350–351. “Otherwise,” the Court explained, a judge “would be denied” the “most satisfactory means” of “reaching a just conclusion” about the patent’s scope—a conclusion needed to resolve the infringement charge. Id.,
at 350–351. “The distinction” thus established, the Court thought, “may be a nice one, but seems to be workable.” Id.,
And, indeed, the Court applied it to decide the case at hand for the assignor, finding that he had not infringed the properly narrowed claim.[2
Finally, the Court left for another day several other questions about the contours of assignor estoppel. One concerned privity: When was an assignor so closely affiliated with another party that the latter would also be estopped? See id.,
at 355. Another related to consideration: What if an assignor had received only a nominal amount of money for transferring the patent? See ibid.
But the question that most interested the Court was whether estoppel should operate differently if the assignment was not of a granted patent but of a patent application—as in fact was true in that case. The Court saw a possible distinction between the two. In a patent application, the Court began, the inventor “swor[e] to” a particular “specification.” Id.,
at 352. But the exact rights at issue were at that point “inchoate”—not “certainly defined.” Ibid.
And afterward, the Court (presciently) observed, the claims might be “enlarge[d]” at “the instance of the assignee” beyond what the inventor had put forward. Id.,
at 353. That might weaken the case for estoppel. But the Court decided not to decide the issue, given its holding that the assignor had not infringed the (narrowed) patent claim anyway.
Minerva’s main argument here, as in the Federal Circuit, is that “assignor estoppel should be eliminated”—and indeed has been already. Brief for Petitioner 17. We reject that view. The doctrine has lasted for many years, and we continue to accept the fairness principle at its core. Minerva’s back-up contention is that assignor estoppel “should be constrained.” Id.,
at 41. On that score, we find that the Federal Circuit has applied the doctrine too expansively. Today, we clarify the scope of assignor estoppel, including in the way Westinghouse
In its quest to abolish assignor estoppel, Minerva lodges three main arguments. The first two offer different reasons for why the doctrine is already defunct: because Congress repudiated it in the Patent Act of 1952 and because, even if not, this Court’s post-Westinghouse
cases “leave no room for the doctrine to continue.” Brief for Petitioner 20. The third, by contrast, is a present-day policy claim: that assignor estoppel “imposes” too high a “barrier to invalidity challenges” and so keeps bad patents alive. Id.,
at 38. (The principal dissent essentially endorses the first two arguments, but not the third. See post,
at 1, 4–6, 11 (opinion of Barrett, J.).)
On the first point, we do not agree that the Patent Act of 1952 abrogated assignor estoppel. The statutory language Minerva relies on provides that “[i]nvalidity” of the patent “shall be [a] defense[ ] in any action involving” infringement.
35 U. S. C. §282(b). According to Minerva, that language “instructs that invalidity must
be available as a defense in every
[infringement] action,” thus “leav[ing] no room for assignor estoppel.” Brief for Petitioner 17–18 (emphasis in original). But to begin with, similar language, entitling a defendant to plead invalidity in any infringement action, was in the patent statute when Westinghouse
was decided. See Patent Act of 1897, ch. 391, §2,
692 (“In any action for infringement the defendant may plead” invalidity). And anyway, Minerva’s view is untenable because it would foreclose applying in patent cases a whole host of common-law preclusion doctrines—not just assignor estoppel, but equitable estoppel, collateral estoppel, res judicata, and law of the case. That broad result would conflict with this Court’s precedents. See, e.g.
, SCA Hygiene Products Aktiebolag
v. First Quality Baby Products, LLC
, 580 U. S. ___, ___ (2017) (slip op., at 16) (recognizing equitable estoppel in a patent suit). And it would subvert congressional design. For Congress “legislate[s] against a background of common-law adjudicatory principles,” and it “expect[s]” those principles to “apply except when a statutory purpose to the contrary is evident.” Astoria Fed. Sav. & Loan Assn.
501 U.S. 104
, 108 (1991) (internal quotation marks omitted). Assignor estoppel was by 1952 just such a background principle of patent adjudication, and Congress gave no indication of wanting to terminate it or disturb its development. Nor has Congress done so since that time.
We likewise do not accept Minerva’s view that two of our post-Westinghouse
decisions have already interred assignor estoppel. According to Minerva (quoting the case’s dissent), Scott Paper Co.
v. Marcalus Mfg. Co.
“eliminated any justification for assignor estoppel and ‘repudiated’ the doctrine.” Reply Brief 13 (quoting
326 U.S. 249
, 264 (1945) (Frankfurter, J., dissenting)). And if that were not enough, Minerva continues, our decision in Lear, Inc.
395 U.S. 653
(1969), also “eviscerated any basis for assignor estoppel.” Reply Brief 13. But we think the words “eliminated,” “repudiated,” and “eviscerated” are far off. Scott Paper
in fact retained assignor estoppel; all they did was police the doctrine’s boundaries (just as Westinghouse
did and we do today).
Whatever a worked-up dissent charged, Scott Paper
did nothing more than decline to apply assignor estoppel in a novel and extreme circumstance. The petitioner in Scott Paper
made the same ask Minerva does here: to abolish the Westinghouse
rule. The Court expressly declined that request. See 326 U. S., at 254. And it restated the “basic principle” animating assignor estoppel, describing it as “one of good faith, that one who has sold his invention may not, to the detriment of the purchaser, deny the existence of that which he has sold.” Id.,
at 251. The Court, to be sure, declined to apply the doctrine in the case before it. There, estoppel would have prevented the assignor from making a device on which the patent had expired—a device, in other words, that had already entered the public domain. The Court could not find any precedent for applying estoppel in that situation. See id.,
at 254. And the Court thought that doing so would carry the doctrine too far, reasoning that the public’s interest in using an already-public invention outweighs the “interest in private good faith.” Id.,
at 256–257. But the Court did not question—again, it reaffirmed—the principle of fairness on which assignor estoppel rests in more common cases, where the assignee is not claiming to control a device unequivocally part of the public domain. See id.,
at 251. In those cases, the doctrine remained intact.
gives Minerva still less to work with. In that case, the Court considered and toppled a different patent estoppel doctrine. Called licensee estoppel, it barred (as its name suggests) a patent licensee from contesting the validity of the patent on a device he was paying to use. Minerva’s basic claim is that as goes one patent estoppel rule, so goes another. Brief for Petitioner 23–25. But Lear
did not purport to decide the fate of the separate assignor estoppel doctrine. To the contrary, the Court stated that the patent holder’s “equities” in the assignment context
“were far more compelling than those presented in the typical licensing arrangement.” 395 U. S., at 664. And so they are. As explained earlier, assignor estoppel rests on the idea that the assignor has made an explicit or implicit representation about the patent’s validity, and received some kind of payment in return. See supra,
at 5–6. No rationale of that kind supports licensee estoppel. The licensee is a buyer of patent rights, not a seller. He has given no assurances of the patent’s worth. All he has done is purchase the right to use a patented device—which, if the patent is invalid, he need not have done. Lear
’s refusal to bar a licensee’s claim of invalidity showed that the Court was alert to “the important public interest in permitting full and free competition in the use of ideas”—and so would not always apply patent estoppel doctrines. 395 U. S., at 670. But that does not mean, at the other extreme, always rejecting those doctrines. Lear
counseled careful attention to the equities at stake in discrete patent contexts—and expressly distinguished assignor from licensee estoppel.
In sum, Scott Paper
where they found it—as a bounded doctrine designed to prevent an inventor from first selling a patent and then contending that the thing sold is worthless. Westinghouse
saw that about-face as unfair; Scott Paper
never questioned that view. At the same time, Westinghouse
realized that assignor estoppel has limits: Even in approving the doctrine, the Court made clear that not every assignor defense in every case would fall within its scope. See supra,
at 7–8. Scott Paper
adopted a similar stance. They maintained assignor estoppel, but suggested (if in different ways) that the doctrine needed to stay attached to its equitable moorings. The three decisions together thus show not the doctrinal “eviscerat[ion]” Minerva claims, Reply Brief 13, but only the kind of doctrinal evolution typical of common-law rules.
Finally, we do not think, as Minerva claims, that contemporary patent policy—specifically, the need to weed out bad patents—supports overthrowing assignor estoppel. In rejecting that argument, we need not rely on stare decisis
: “[C]orrect judgments have no need for that principle to prop them up.” Kimble
v. Marvel Entertainment, LLC
576 U.S. 446, 455 (2015). And we continue to think the core of assignor estoppel justified on the fairness grounds that courts applying the doctrine have always given. Assignor estoppel, like many estoppel rules, reflects a demand for consistency in dealing with others. See H. Herman, The Law of Estoppel §3 (1871) (“An estoppel is an obstruction or bar to one’s alleging or denying a fact contrary to his own previous action, allegation or denial”). When a person sells his patent rights, he makes an (at least) implicit representation to the buyer that the patent at issue is valid—that it will actually give the buyer his sought-for monopoly.[3
] In later raising an invalidity defense, the assignor disavows that implied warranty. And he does so in service of regaining access to the invention he has just sold. As the Federal Circuit put the point, the assignor wants to make a “representation at the time of assignment (to his advantage) and later to repudiate it (again to his advantage).” Diamond Scientific
848 F. 2d, at 1224; see supra,
at 4. By saying one thing and then saying another, the assignor wants to profit doubly—by gaining both the price of assigning the patent and the continued right to use the invention it covers. That course of conduct by the assignor strikes us, as it has struck courts for many a year, as unfair dealing—enough to outweigh any loss to the public from leaving an invalidity defense to someone other than the assignor.[4
Still, our endorsement of assignor estoppel comes with limits—true to the doctrine’s reason for being. Just as we guarded the doctrine’s boundaries in the past, see supra,
at 7–8, 11–13, so too we do so today. Assignor estoppel should apply only when its underlying principle of fair dealing comes into play. That principle, as explained above, demands consistency in representations about a patent’s validity: What creates the unfairness is contradiction. When an assignor warrants that a patent is valid, his later denial of validity breaches norms of equitable dealing. And the original warranty need not be express; as we have explained, the assignment of specific patent claims carries with it an implied assurance. See supra,
at 13. But when the assignor has made neither explicit nor implicit representations in conflict with an invalidity defense, then there is no unfairness in its assertion. And so there is no ground for applying assignor estoppel.
One example of non-contradiction is when the assignment occurs before an inventor can possibly make a warranty of validity as to specific patent claims. Consider a common employment arrangement. An employee assigns to his employer patent rights in any future inventions he develops during his employment; the employer then decides which, if any, of those inventions to patent. In that scenario, the assignment contains no representation that a patent is valid. How could it? The invention itself has not come into being. See Lemley, Rethinking Assignor Estoppel, 54 Houston L. Rev. 513, 525–527 (2016). And so the employee’s transfer of rights cannot estop him from alleging a patent’s invalidity in later litigation.
A second example is when a later legal development renders irrelevant the warranty given at the time of assignment. Suppose an inventor conveys a patent for value, with the warranty of validity that act implies. But the governing law then changes, so that previously valid patents become invalid. The inventor may claim that the patent is invalid in light of that change in law without contradicting his earlier representation. What was valid before is invalid
today, and no principle of consistency prevents the assignor from saying so.
Most relevant here, another post-assignment development—a change in patent claims—can remove the rationale for applying assignor estoppel. Westinghouse
itself anticipated this point, which arises most often when an inventor assigns a patent application, rather than an issued patent. As Westinghouse
noted, “the scope of the right conveyed in such an assignment” is “inchoate”—“less certainly defined than that of a granted patent.” 266 U. S., at 352–353; see supra,
at 9. That is because the assignee, once he is the owner of the application, may return to the PTO to “enlarge[ ]” the patent’s claims. 266 U. S., at 353;
35 U. S. C. §120; 37 CFR §1.53(b). And the new claims resulting from that process may go beyond what “the assignor intended” to claim as patentable. 266 U. S., at 353. Westinghouse
did not need to resolve the effects of such a change, but its liberally dropped hints—and the equitable basis for assignor estoppel—point all in one direction. Assuming that the new claims are materially broader than the old claims, the assignor did not warrant to the new claims’ validity. And if he made no such representation, then he can challenge the new claims in litigation: Because there is no inconsistency in his positions, there is no estoppel. The limits of the assignor’s estoppel go only so far as, and not beyond, what he represented in assigning the patent application.
The Federal Circuit, in both its opinion below and prior decisions, has failed to recognize those boundaries. Minerva (recall, Truckai’s alter-ego) argued to the court that estoppel should not apply because it was challenging a claim that was materially broader than the ones Truckai had assigned. But the court declined to consider that alleged disparity. Citing circuit precedent, the court held it “irrelevant” whether Hologic had expanded the assigned claims: Even if so, Minerva could not contest the new claim’s validity. 957 F. 3d, at 1268 (quoting Diamond Scientific
, 848 F. 2d, at 1226); see supra
, at 4. For the reasons given above, that conclusion is wrong. If Hologic’s new claim is materially broader than the ones Truckai assigned, then Truckai could not have warranted its validity in making the assignment. And without such a prior inconsistent representation, there is no basis for estoppel.
We remand this case to the Federal Circuit to now address what it thought irrelevant: whether Hologic’s new claim is materially broader than the ones Truckai assigned. The parties vigorously disagree about that issue. In Truckai’s view, the new claim expanded on the old by covering non-moisture-permeable applicator heads. In Hologic’s view, the claim matched a prior one that Truckai had assigned. Resolution of that issue in light of all relevant evidence will determine whether Truckai’s representations in making the assignment conflict with his later invalidity defense—and so will determine whether assignor estoppel applies.
This Court recognized assignor estoppel a century ago, and we reaffirm that judgment today. But as the Court recognized from the beginning, the doctrine is not limitless. Its boundaries reflect its equitable basis: to prevent an assignor from warranting one thing and later alleging another. Assignor estoppel applies when an invalidity defense in an infringement suit conflicts with an explicit or implicit representation made in assigning patent rights. But absent that kind of inconsistency, an invalidity defense raises no concern of fair dealing—so assignor estoppel has no place.
For these reasons, we vacate the judgment of the Federal Circuit and remand the case for further proceedings consistent with this opinion.
It is so ordered.