In Teva Pharmaceuticals USA, Inc. v. Sandoz, Inc., the Supreme Court revised the standard of review used by the Federal Circuit for nearly twenty years in reviewing claim construction rulings, replacing a de novo standard with a “clearly erroneous” standard. Teva sued Sandoz for infringing its patent covering a method of manufacturing Copaxone, a drug used to treat multiple sclerosis. The relevant claim recited that the active ingredient has a particular “molecular weight.” Sandoz argued that the claim was indefinite under 35 U.S.C. §112 ¶2 because the term “molecular weight” could refer to any of three different methods of calculating molecular weight.

After hearing expert testimony, the District Court found that the patent was sufficiently definite because a skilled artisan would understand the term “molecular weight” referred specifically to one of the three allegedly possible interpretations. The District Court’s decision was based on the testimony of Teva’s expert regarding how one skilled in the art at the time of the invention would have understood the term. On appeal, the Federal Circuit reviewed the District Court’sdecision, including the evaluation of expert testimony, under the de novo standard and found that the claims were indefinite. The Supreme Court reversed the Federal Circuit’s decision, finding that the Federal Circuit applied the incorrect standard of review.