Infringement Defenses

Assignor Estoppel Doctrine still with us (updated 06/30/21)

Assignor Estppel is “an ecquitable doctrine that prevents one who has assigned the rights to a patent (or patent application) from later contending that what was assigned is a nullity.” Diamond Sci.1)

The Supreme Court, in a 5-4 vote, decided to keep Assignor Estoppel with us a while longer, but stated that the Federal Circuit has applied the doctrine too broadly. The crux of the ruling is this: “The doctrine applies when, but only when, the assognor's claim of invalidity contradicts explicit or implicit representations he made in assigning the patent.”

It is worth noting that the Supreme Court was apparently influenced by Mark Lemly's amicus brief, and in fact cited Lemly's Houston Law Review article, “Rethinking Assignor Estoppel2)) (private cache) in two places in the majority opinion.

Procedural History

Inventor Csaba Truckai of two Hologic patents (6,872,183 and 9,095,348) initially assigned the invention (priority applications) to a company he co-founded called “NovaCept, Inc.” NovaCept was later acquired by Cytyc Corporation and Hologic later acquired Cytyc. The following year, Truckai left NovaCept and founded the accused infringer, Minerva, who filed IPR petitions challenging the patentability of the patents. The Board held that the '183 patents are unpatentable as obvious. Hologic moved for summary judgment that the doctrine of assignor estoppel bars Minerva from challenging the validity of the '183 and '348 patent claims, which the district court granted. The Federal Circuit upheld the IPR decisions on appeal. The district court stated that the ~$5M judgment against Minerva is not affected by the IPR or the Circuit's decision on appeal because of the findings of assignor estoppel.

In Hologic, Inc. v. Minerva Surgical, Inc., the Federal Circuit held:

  • The primary justification for the doctrine is “unfairness and injustice.”
  • The court has limited the doctrine in certain instances:
    • “Although estopped parties cannot challenge the validity of the patent at issue, assignor estoppel does not limit their ability to defend themselves in other ways, including arguing that the patentee is itself collaterally estopped from asserting a patent found invalid in a prior proceeding.”
    • Also, “an estopped party may argue for a narrow claim construction, or that the accused devices are within the prior art and therefore cannot infringe.”
  • In this case, assignor estoppel does not preclude Minerva from relying on the Hologic decision (that upheld the IPR invalidity) to argue that the '183 claims are void ab initio. Thus, [. . ..] Minerva had the right to [file the IPR] under the AIA and this court's precedent [in Arista Networks Inc. v. Cisco Sys., Inc. 3)].“
  • In Diamond Scientific, we recognized that some courts questioned the vitality of the assignor estoppel doctrine following the Supreme Court’s decision abolishing licensee estoppel in Lear, Inc. v. Adkins, 4). We concluded, however, that nothing in Lear eliminated assignor estoppel.

Supreme Court Review

Oral Hearing

As reported by Law360, in oral arguments on April 21, 2021, Justices Kavanaugh and Sotomayor expressed skepticism of disturbing such a “well settled” legal principle.


Background: On January 8, 2021, the Supreme Court granted certiorari for review on the question:

Whether a defendant in a patent infringement action who assigned the patent, or is in privity with an assignor of the patent, may have a defense of invalidity heard on the merits.

The petition for writ attracted many interested amici, including a brief submitted by Mark Lemley of Stanford University, who argued that the Federal Circuit's interpretation is broader than Supreme Court precedent, and needs to be narrowed. Specifically, the Federal Circuit's precedent:

  1. “reaches well beyond the assignor to bar validity charges by a wide range of parties in privity with assignors”
  2. “extends beyond cases where an inventor knowingly and voluntarily transfers a patent” and “is not limited to cases where an inventor or assignor misrepresents a patent's validity, or indeed makes any representation or warranty at all about the patent”
  3. “applied assignor estoppel where the assignee amended the claims in a patent after assignment to cover things the inventor never intended” noting ”that is what happened here“ (emphasis added).

Supreme Court Holdings:

Minverva Surgical Inc. v. Hologic Inc. (booklet) (case 20-440) was decided June 29, 2021. Five Justices, led by Kagan and joined by Roberts, Breyer, Sotomayor, and Kavanaugh, wrote the following holdings. (In the below quotes, citations are removed.)

  • Assignor Estoppel . . . 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.
  • [T]he Patent Act of 1952 [did not abrogate] assignor estoppel. [. . .] We likewise do not accept Minerva's view that two of our post-Westinghouse decisions have already interred assignor estoppel.
    • Scott Paper did nothing more than decline to apply assignor estoppel in a novel and extreme circumstance [in which the court declined to enforce assignor estoppel with respect to an expired patent].
    • [T]he Court stated [in Lear, which related to licensee estoppel] that the patent holder's “equities” in the assignment context “were far more compelling than those presented in the typical licensing arrangement.”
  • [C]ontemporary patent policy–specifically, the need to weed out bad patents–[does not support] overthrowing assignor estoppel.
  • What creates unfairness is contradiction. When an assignor warrants that a patent is valid, his later denial of validity breaches norms of equitable dealing. [. . .] But when the assignor has made neither explicit nor implicit representations. . . then there is no unfairness in its assertion.
    • 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. 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.
    • 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. Most relevant here . . . a change in patent claims can remove the rationale for applying assignor estoppel. [. . .] As Westinghouse noted, “the scope of the right conveyed in such an assignment” is “inchoate”–“less certainly defined than that of a granted patent.” That is because thee assignee, once he is the owner of the application, may return to the PTO to “enlarge[]” the patent's claims. And the new claims resulting from that process may go beyond what “the assignor intended” to claim as patentable. [. . .] The limits of the assignor's estoppel go only so far as, and not beyond, what he represented in assigning the application.


Justice Alito wrote a dissent focusing on the “evasion” of stare decisis by the Court and the principal dissent. Stated Alito: “I do not think we can decide the question that the petition in this case presents unless we decide whether Westinghouse should be overruled. Because the majority and the principal dissent refuse to decide whether Westinghouse should be overruled, I would dismiss the writ as improvidently granted.” In apparent response to this argument, the Majority held, “we do not think . . . 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 judments have no need for that principle to prop them up.' Kimble v. Marvel Entertainment, LLC.

Justice Barrett, joined by Thomas and Gorsuch wrote a further dissent that argued that “this case turns on whether the Patent Act of 1952 incorporates the doctrine [of assignor estoppel], and held that the doctrine of assignor estoppel, previously held as being incorporated into the Patent Act of 1870, was not carried over into the Patent Act of 1952.



Alito Barrett Breyer Gorsuch Kagan Kavanaugh Roberts Sotomayor Thomas
Op., Part I; history of patent administration and this case Joined Joined Joined Joined Authored
Op., Part II; precedent of Edmund; failures of Thomas' dissent; “the unreviewable authority wielded by APJs during [IPR] is incompatible with their appointment by the Secretary as inferior officers [under the Appointments Clause]. Joined Joined Joined Joined Authored
Op., Part III; Joined Joined Authored

848 F.2d at 1224
54 Houston L. Rev. 513 (2016
908 F.3d 792, 804 (Fed. Cir. 2018
395 U.S. 653, 666 (1969)

User Tools