The radiosensitive mutant xrs-6, derived from Chinese hamster ovary cells, is defective in DNA double-strand break repair and in ability to undergo V(D)J recombination. The human XRCC5 DNA repair gene, which complements this mutant, is shown here through genetic and biochemical evidence to be the 80-kilodalton subunit of the Ku protein. Ku binds to free double-stranded DNA ends and is the DNA-binding component of the DNA-dependent protein kinase. Thus, the Ku protein is involved in DNA repair and in V(D)J recombination, and these results may also indicate a role for the Ku-DNA-dependent protein kinase complex in those same processes.