The US Court of Appeals for the Federal Circuit reiterated that while assignor estoppel prevents a party that assigned a patent to another party from later challenging the validity of the assigned patent in district court, it does not preclude the party from challenging the validity of the assigned patent in an America Invents Act inter partes review (IPR) proceeding. Hologic, Inc. v. Minerva Surgical, Inc., Case Nos. 19-2054; -2081 (Fed. Cir. Apr. 22, 2020) (Stoll, J.) (Stoll, J., additional views).