The X-ray repair cross-complementing protein XRCC4 and DNA Ligase IV are essential for repairing double-strand breaks in DNA. These stories form a critical complex composed of two molecules of each protein that preferentially bind DNA with nicks or broken ends. As an obligatory accessory molecule, XRCC4 binds to DNA Ligase IV and enhances its joining activity. The XRCC4 / DNA Ligase IV complex is also involved in V (D) J recombination. V (D) (in) a pair of high incidence of apoptosis in the development nervous system and a block in B and T cell maturation.