The XRCC5 gene encodes an 80 kDa protein that plays a key role in the nonhomologous end joining (NHEJ) DNA repair pathway. The NHEJ repair pathway is essential for DNA double-strand break repair and V(D)J recombination, a unique genetic recombination mechanism that occurs only in developing lymphocytes during the early stages of T and B cell maturation.
The Ku80 protein carries out its function by forming a heterodimer with the Ku70 protein encoded by the XRCC6 gene. This heterodimer, known as the Ku protein or the ATP-dependent DNA helicase II, is a basket-shaped molecule that slides onto the DNA end and translocates inward. It acts as a regulatory subunit of the DNA-dependent protein kinase complex (DNA-PK) by increasing the affinity of the catalytic subunit (PRKDC) to DNA.