The gene encoding the catalytic subunit of DNA-dependent protein kinase (DNA-PKcs) has been proposed recently as a candidate gene for the mouse severe combined immune deficiency (scid) locus, We have used a partial cDNA clone for human DNA-PKcs to map the mouse homologue using a large interspecific backcross panel, We found that the mouse gene for DNA-PKcs does not recombine with scid, consistent with the hypothesis that scid is a mutation in the mouse gene for DNA-PKcs.