An article from Fenwick & West LLP on 27 August reported on a recent US Supreme Court case which addressed the situation where an inventor develops an invention, files a patent application and assigns the application for value to the company they founded. Then later, the inventor founds another company and develops an improved version of their original invention. The inventor’s former company sues the inventor or the inventor’s current company for patent infringement. In response, the inventor argues that the patent for the original invention is invalid. Should the inventor be able to do so? The case involved the doctrine of “assignor estoppel” — which bars the inventor assignor of a patent from denying the validity of their previously assigned patent.