Zhang and fellow Broad researcher George Church have worked on another strategy to combat off-target mutations, too. Cas9 is often compared to a pair of scissors, but it’s actually two pairs of scissors fused together, each of which cuts one of DNA’s two strands. Zhang and Church have mutated Cas9 to blunt one of those scissors, so it only cuts one strand. Now you need a second Cas9 with second guide RNA to cut the second strand—with redundancy comes less erro