One of the most celebrated tools of synthetic biology goes by the name Clustered Regularly Interspaced Short Palindromic Repeats, or CRISPR. Explore the basic biology of CRISPR, which, together with an associated enzyme called Cas9, can target and alter specific sequences of DNA. Heralding a revolution in medicine and other fields, CRISPR borders on the miraculous, but it is also ethically fraught.