Athabascan SCID (SCIDA) is an autosomal recessive disorder found among Athabascan-speaking Native Americans and is manifested by the absence of both T and B cells (T− B− NK+ SCID). We previously mapped the SCIDA gene to a 6.5-cM interval on chromosome 10p. SCIDA fibroblasts were found to have defective coding joint and reduced, but precise signal joint formation during V (D) J recombination. After excluding potential candidate genes, we conducted a combined positional candidate and positional cloning approach leading to the identification of nine novel transcripts in the refined SCIDA region. One of the transcripts showed significant homology with the mouse and yeast SNM1/PSO 2 and was recently reported (Artemis) to be responsible for another T− B− NK+ SCID condition (radiation sensitive SCID) in 13 patients of primarily European origin. In our evaluation of this gene, we have identified a unique nonsense mutation in 21 SCIDA patients that is closely correlated to the founder haplotypes that we had previously identified. This nonsense founder mutation results in the truncation of the deduced protein product. The wild-type construct of the primary transcript can effectively complement the defective coding joint and reduced signal joint formation in SCIDA fibroblasts. The above results indicate that this SNM1-like gene (Artemis) is the gene responsible for SCIDA. We also discovered three additional alternative exons and detected at least six alternatively spliced SCIDA variants (SCIDA-V1, 2, 3, 4, 5, and 6) coexisting with the primary transcript in trace amounts. Finally, we found that the SCIDA primary transcript (Artemis) encodes a nuclear protein.