The popular CRISPR-Cas9 gene editing system gets a significant speed boost from a new technique demonstrated by Yang Liu and colleagues, which regulates the process's crucial DNA cleavage step with light. The result, which Liu and colleagues call very fast CRISPR, reduces the cleavage time down from hours to seconds. The fast system allows researchers to study the very earliest molecular steps in DNA repair at high resolution, and to control genetic editing at the single allele level. During CRISPR-Cas9 gene editing, the Cas9 enzyme is the "scissors" that snips DNA at the spot where genetic material will be edited. A guide RNA molecule allows Cas9 to bind to the DNA at the correct spot, but this process isn't an immediate one, sometimes taking hours. To better control Cas9's action, Liu et al. altered part of the guide RNA sequence with light-sensitive nucleotides, so that it could bind Cas9 to its DNA target but keep the enzyme from cutting until the guide RNA was exposed to light. With this "caged" RNA, the researchers can control cleavage over a timescale of seconds to watch DNA strand repair from its start. The process is also so precise that it allows one allele of a gene to be edited at a time, offering a way to create heterozygous mutations for studying complex genetic traits. The technique could help transform CRISPR-Cas9 editing from a blunt to a precision instrument, Maria Jasin notes in a related Perspective.