The enzymes that can cut DNA or RNA are called “nucleases.” In order to build a system for gene editing, researchers needed a nuclease that could be instructed to cut any sequence that the researchers chose to target. By 2000, they had found a tool to do this. The FokI enzyme, which is found in some soil and pond bacteria, has two domains: one that serves as scissors that can cut DNA and another that serves as a guide telling it where to go. These domains can be separated, and the first can be reprogrammed to go anywhere the researchers want.1
