Not since the discovery of RNA interference has a new technology made such a powerful impact in the molecular biosciences. The RNA-guided, modified type II prokaryotic clustered regularly interspaced palindromic repeats system (CRISPR) represents not only an exceptional tool but also potentially an important new therapeutic modality. CRISPR, in its native function, provides adaptive immunity in bacteria by introducing targeted DNA mutations in pathogenic viruses and plasmids (reviewed in ref. 1). The breakthrough technology came in 2012 with the development of modified CRISPR components that worked in human cells. This simplified system was found to be easily adapted into expression vectors to target any DNA sequence from virtually any organism, thereby greatly expanding its function and utility. To date this system has been used in human cells to target gene excision, activating and repressing proteins, as well as to instill gene fusions.