It has been shown that several X-ray-sensitive Chinese hamster cell mutants defective in repair of DNA double-strand breaks (DSBs) are also impaired in the process of V(D)J recombination. The hamster mutants with this phenotype represent three distinct complementation groups, represented by the xrs series, XR-1 and V-3. The murine scid cell line also shows the same phenotype, and therefore we examined whether the scid mutant represents a new complementation group or belongs to one of the existing groups. Scid cells were fused with hamster cell mutants representing the three complementation groups. Hybrids between V-3 and scid cells were only partially complemented for X-ray sensitivity, whereas hybrids derived from fusions with the other mutants were resistant to X rays. These results suggest that V-3 and scid cells are defective in the same gene. To confirm this finding, a single human chromosome 8, which is known to carry the scid gene, was introduced into V-3 cells by microcell-mediated chromosome transfer. Nine hybrid clones derived from V-3 and carrying human chromosome 8 were obtained, and seven were found to be partially complemented for X-ray sensitivity. When human chromosome 8 was introduced into scid cells, seven of eight hybrid clones became resistant to X rays. The results indicate that the defective genes in V-3 and scid are both localized on human chromosome 8. This supports the results from the fusion analysis that V-3 and scid cells are defective in the same gene.