This decision refuses to overrule the assignor estoppel doctrine which prevents the assignor of a patent from challenging the patent’s validity if doing so would contradict the assignor’s express or implied warranty that the patent was good at the time the assignor sold it. However, the estoppel does not apply when the assignment is made before a patent application is even filed (as is often the case with employees’ assignment of patentable items to their employer) or where post-assignment events change either the applicable law (such as a post-assignment decision reinterpreting patent law) or, as in this case, an assignment of a pending patent application which the assignee then broadens–the assignor can challenge the validity of the broadened patent claims.