Traditionally, the doctrine of assignor estoppel prevented a seller of a patent from challenging the validity of that patent. One situation where I have seen this arise is when an inventor assigns his/her rights in a patent to his/her employer in exchange for consideration, and subsequently, the inventor leaves that employer to start a competing business that performs activities that may be within the scope of the previously assigned patent. Although the doctrine of assignor estoppel is still alive and well in the federal courts, yesterday the PTAB deemed its decision in Athena Automation Ltd. v. Husky Injection Molding Systems Ltd. last fall to be precedential (required votes from a majority of the PTAB’s voting members plus the USPTO director). In Athena, the PTAB held that the AIA overruled the doctrine of assignor estoppel within the context of inter partes reviews (IPRs).  Consequently, the PTAB found that anyone other than a patent’s current owner can challenge the patent’s validity in an IPR.  Thus, the seller of a patent including a named inventor of the patent can challenge the validity of that patent in an IPR proceeding before the PTAB. It should be appreciated that the Federal Circuit previously found the decision in Athena to be exempt from judicial appeal, so this precedential opinion from the PTAB should stick.