The DNA rearrangement that assembles antibody and T cell receptor genes is beginning to be understood. New work shows that the first step is a site-specific cleavage which can be performed by purified RAG1 and RAG2 proteins. The later steps largely overlap with the repair of radiation-induced DNA double-strand breaks, as indicated by the identity of several newly cloned factors. These developments open the way for a thorough biochemical study of V(D)J recombination.