The district court agreed that assignor estoppel barred Minerva's defense and the Federal Circuit affirmed in part. Minerva filed a writ. In a 5-4 decision, Justice Kagan reasoned that courts have long applied the doctrine of assignor estoppel to deal with inconsistent representations about patent validity.
The Court also recognized limits to assignor estoppel: (1) assignment occurs before inventor can make warranty of validity to patent claims, e.g., employee assigning rights in future inventions (2) later legal development renders the warranty as irrelevant (3) post-assignment change in patent claims, e.g., inventor assigns patent application and later to broaden the scope of claims.
Full text of the opinion is available here.