Assignor Estoppel[Patent law] Law and Legal Definition

Assignor estoppel means prohibiting a patent's seller (assignor) from attacking the patent's validity in subsequent patent infringement litigation. Assignor estoppel is based on the doctrine of legal estoppel in which a grantor is prohibited from challenging the validity of his/her patent grant. The policies applicable to assignor estoppel are different from those applicable to licensee estoppel. Assignor estoppel prevents unfairness and injustice of permitting a party to sell patent rights and later making statements that whatever sold was worthless.

[ Mentor Graphics Corp. v. Quickturn Design Sys., 150 F.3d 1374 (Fed. Cir. 1998)]