The downside is that these single-scissor Cas9s can still “nick” DNA individually and cause potential mutations. So yet another group led by Harvard’s Keith Joung have fused the guide RNA-binding part of Cas9 to the the scissors of another a DNA-cutting protein called FokI. Not only do you need two FokI-Cas9 to cut a whole piece of DNA, but the two individual hybrid proteins need to actually combine into one mega protein before either will cut DNA, so you don’t get any nicking either.