June 29, 2021

Supreme Court Upholds and Limits Assignor Estoppel in Patent Cases

4 min

Assignor estoppel is a common-law doctrine that prevents the assignor of a patent from challenging the validity of the assigned patent in district court litigation.  On June 29, 2021, the Supreme Court in a 5-4 decision, Minerva Surgical, Inc. v. Hologic, Inc., No. 20-440, upheld the doctrine, but limited its application to situations in which the invalidity challenge contradicts an explicit or implicit representation that the assignor made in assigning the patent.  Under Minerva, assignor estoppel will not apply in situations where a patent application is assigned, and the claims that issue from the application are “materially broader” than the assigned claims.

The Court’s Opinion

Justice Kagan wrote the opinion of the Court, which Roberts, Breyer, Sotomayor, and Kavanaugh joined.

The Court rejected Petitioner Minerva’s argument that assignor estoppel should be abolished outright.  In rejecting that argument, the Court explained that the assignor estoppel doctrine is grounded in “centuries-old fairness principles” that prevent an assignor from attacking the validity of a right he had formerly sold; that the Court had unanimously endorsed assignor estoppel in Westinghouse Elec. & Mfg. Co. v. Formica Insulation Co., 266 U.S. 342 (1924), and had maintained the doctrine in Scott Paper Co. v. Marcalus Mfg. Co., 326 U.S. 249 (1945) and Lear, Inc. v. Adkins, 395 U.S. 653 (1969); and that abolishing assignor estoppel would “foreclose applying in patent cases a whole host of common-law preclusion doctrines.”

The Court, however, concluded that “the Federal Circuit has applied the doctrine too expansively” to situations where there was no contradiction between the assignor’s warranty at the time of assignment and its later invalidity challenge.  The Court observed that, “[w]hen 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. 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.”

The Court provided three examples to which assignor estoppel would not apply:

  • “[W]hen the assignment occurs before an inventor can possibly make a warranty of validity as to specific patent claims,” e.g., when an employee prospectively assigns to his employer patent rights in any future inventions he develops during his employment.
  • “[W]hen a later legal development renders irrelevant the warranty given at the time of assignment,” e.g., when the law changes following assignment, such that “previously valid patents become invalid.”
  • “[W]hen an inventor assigns a patent application, rather than an issued patent” and the new claims that issue from the patent application are “materially broader” than the old claims.

The Court remanded to the Federal Circuit to address whether the claim asserted by Hologic against Minerva is “materially broader” than the claims assigned to Hologic by Minerva’s founder, Csaba Truckai.  According to the Court, “[i]f 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.”


Justice Barrett wrote the principal dissent, which Justices Thomas and Gorsuch joined.  Justice Barrett asserted that the assignor estoppel doctrine was abrogated by the Patent Act of 1952, which states that invalidity “shall” be a defense “in any action involving the validity or infringement of a patent,” and provides no exceptions for actions in which an assignor is the defendant.  Justice Barrett also asserted that the assignor estoppel doctrine was not so well settled as to support a presumption that Congress had endorsed the doctrine when it adopted the Patent Act of 1952.

Justice Alito dissented separately, asserting that Minerva could not properly be decided without first determining whether Westinghouse should be overruled.  Justice Alito also criticized the principal dissent for adopting “an ill-suited standard for determining whether one of our precedents has been abrogated by Congress” (emphasis in original).  According to Justice Alito, “[s]uggesting that a rule announced in a decision of this Court can cease to be a precedent if it is not ‘well settled’ is very strange.”