The dispute over who owns the intellectual property rights to the game-changing genome editing system CRISPR-Cas heated up in January after the US Patent and Trademark Office's Patent Trial and Appeal Board said it would decide the fate of the patents in a trial-like procedure called an interference proceeding. The first patent for the CRISPR-Cas system was granted in April 2014 to Feng Zhang, a bioengineer at the Broad Institute and founder of Editas Medicine, both in Cambridge, Massachusetts (Nat. Biotechnol. 32, 599–601, 2014). Zhang's patent application was preceded by one submitted by Jennifer Doudna, a biochemist at the University of California, Berkeley, and her co-inventors. Attorneys for both parties have been battling ever since. Their arguments center on who first demonstrated a method for editing a mammalian genome using CRISPR-Cas. The US patent office operates under a first-to-file system for granting patents, but Zhang and Doudna's applications are being judged under the old first-to-invent rules owing to their filing and priority dates. Doudna's attorneys in April 2015 requested the interference proceeding against Zhang's patent, number 8697359, and nine other CRISPR-related patents he has since filed. The contest was recommended by a patent office examiner on December 21, 2015. The Patent Trial and Appeal Board on January 11, 2016 gave the go-ahead for the interference proceeding in a way that evens the playing field for both parties. Such contests are often lengthy and can involve testimony from inventors.