By Aron Fischer and Rachel Schwartz ( October 5, 2017, 12:06 PM EDT) -- In July, a split panel of the Federal Circuit upheld the district court's use of an adverse inference from litigation misconduct to hold a patent unenforceable for inequitable conduct. The Federal Circuit's decision in the case, Regeneron Pharmaceuticals Inc. v. Merus NV,[1] raises interesting questions about the relationship between attorney misconduct during litigation (which is not supposed to affect the enforceability of a patent) and misconduct during prosecution of the patent (which can). Because the court's opinion gives no clear answer to these questions, it opens new tactical opportunities for defendants asserting inequitable conduct defenses in patent cases and may incrementally expand the use of a doctrine that the Federal Circuit has famously referred to as a "plague"[2] and repeatedly tried to rein in....