The Chinese hamster ovary xrs mutants are sensitive to ionizing radiation, defective in DNA double-strand break rejoining, and unable to carry out V(D)J recombination effectively. Recently, the gene defective in these mutants, XRCC5, has been shown to encode Ku80, a component of the Ku protein and DNA-dependent protein kinase. We present here a YAC contig involving 25 YACs mapping to the region 2q33-q34, which encompasses the XRCC5 gene. Eight new markers for this region of chromosome 2 are identified. YACs encoding the Ku80 gene were transferred to xrs cells by protoplast fusion, and complementation of all the defective phenotypes has been obtained with two YACs. We discuss the advantages and disadvantages of this approach as a strategy for cloning human genes complementing defective rodent cell lines.