Genome editing + optogenetics = very fast CRISPR (vfCRISPR). Two revolutionary techniques meet in the paper by Yang Liu and colleagues just published in Science. The Johns Hopkins University team developed a caged RNA strategy that allows Cas9 to bind DNA but needs light at wavelengths that are not phototoxic to activate cleavage. The cut is immediate upon light exposure, offering scientists a way to study DNA repair from its start. The process is so precise that one allele of a gene can be edited at a time, allowing the generation of heterozygous mutations for studying complex genetic traits. See also the perspective by Darpan Medhi and Maria Jasin in Science.