The NHEJ pathway is initiated in response to the formation of a DNA double-strand break (DSB) induced by a DNA-damaging agent such as ionizing radiation. First, the Ku70/80 heterodimer binds to the ends of the DSB. The catalytic subunit of the DNA-dependent protein kinase (DNA-PKcs) is then recruited to DNA-bound Ku to form the DNA-PK holoenzyme. The ends of the break are brought together as two molecules of DNA-PK (one at each end of the break) are joined in a synaptic complex. Following the formation of the synaptic complex, the XRCC4/DNA ligase IV complex is recruited. Prior to end rejoining by ligation, repair protein factors must be removed from the DNA. Both Mg-ATP and the protein kinase activity of DNA-PKcs are required for NHEJ. NHEJ has been suggested as the main pathway for rejoining of IR-induced DSBs in vertebrates. The involvement of homologous recombination repair (HRR) has been postulated to occur only after initial rejoining by NHEJ.