Assignor Estoppel Does Not Apply to IPRs, PTAB Litigation Blog

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Assignor estoppel is a long-established, common-law doctrine of equity, which prevents a party who assigns a patent to another from later challenging the validity of the assigned patent. The doctrine also applies to parties in privity with an estopped assignor, such as a corporation founded by the assignor. On November 9 however, the Federal Circuit (the "Court") handed down a decision in Arista Networks, Inc. v. Cisco Sys., Inc., 17-1525, which says that assignor estoppel does not apply to Inter Parte Review (IPR) proceedings.

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