In the case of Arista Networks, Inc. v. Cisco Sys., Inc. (Federal Circuit, November 9, 2018), the Court of Appeals for the Federal Circuit addressed the issue of whether assignee estoppel applies to prevent an inventor/assignor from later challenging a patent in an inter partes review (an “IPR”) proceeding of the patent at the USPTO.  The traditional rule is that an assignee of a patent (such as an inventor who assigns their rights in a patent) is estopped from later challenging the validity of the patent in litigation.  However, the Court of Appeals for the Federal Circuit found that broad language in the American Invents Act supported allowing any person who is not the current owner of a patent to file an IPR – even if that person is an inventor who assigned their rights in the patent.