RAG1 and RAG2 are essential components of the enzyme complex that initiates assembly of antigen receptor genes from their component V, D, and J gene segments in both B and T cells. The RAG complex recognizes and cleaves specific DNA elements known as recombination signal sequences (RSSs) that flank rearranging gene segments. Subsequently, additional proteins are recruited to mediate joining of the broken DNA ends. This process is essential to generate the diverse array of antigen receptors and thus for the function of lymphocytes in adaptive immunity.