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. 

The XRCC5 gene is located on the long arm of chromosome 2.  It is made up of about 98 kb and has a coding sequence spanning 21 exons.  The protein encoded by the Ku80 gene is approximately 82 kDa in size. It consists of 732 amino acids.