The U.S. Court of Appeals for the Federal Circuit upheld a lower court's ruling that Regeneron Pharmaceuticals Inc.'s '018 patent is unenforceable due to the Tarrytown, N.Y., company's inequitable conduct in prosecuting the patent, which relates to using large DNA vectors to target and modify endogenous genes and chromosomal loci in eukaryotic cells. A practical use of the technology is to target and modify specific genes in mice so the mice develop antibodies that can be used by humans.