Two Artemis-deficient (mArt(-/-)) mouse models, generated independently on 129/SvJ backgrounds, have the expected T(-)B(-)NK(+) severe combined immune deficiency (SCID) phenotype but fail to mimic the human disease because of CD4(+) T cell leakiness. Moreover, immune reconstitution after hematopoietic stem cell transplantation is achieved more readily in these leaky mouse models than in Artemis-deficient humans. To develop a more clinically relevant animal model, we backcrossed the mArt(-/-) mutation onto the C57Bl/6 (B6) background (99.9%), which resulted in virtually no CD4(+) T cell leakiness compared with 129/SvJ mArt(+/-) mice (0.3% +/- 0.25% vs 19.5% +/- 15.1%, P < .001). The nonleaky mouse also was uniquely resistant to engraftment using allogeneic mismatched hematopoietic stem cells, comparable to what is seen in human Artemis deficiency. The genetic background also influenced Artemis-associated radiation sensitivity, with differing degrees of x-ray hypersensitivity evident in 129/SvJ and B6 backgrounds with both the mArt(-/-) and mArt(+/-) genotypes. Our results indicate that immunogenic and DNA repair phenotypes associated with Artemis deficiency are significantly altered by genetic background, which has important implications for the diagnosis and treatment of SCID. Moreover, the B6 mArt(-/-) mouse provides a more accurate model for the human disease and a more appropriate system for studying human Artemis deficiency and for developing improved transplantation and gene therapy regimens for the treatment of children with SCID.