Severe combined immunodeficiency disease (SCID) can be immunologically classified by the absence or presence of T, B, and natural killer (NK) cells. About 30% of T⁻B⁻NK⁺ SCID patients carry mutations in the recombination activating genes (RAG). Some T⁻B⁻NK⁺ SCID patients withoutRAG gene mutations are sensitive to ionizing radiation, and several of these radiosensitive (RS) SCID patients were recently shown to have large deletions or truncation mutations in theArtemis gene, implying a role for Artemis in DNA double-strand break (dsb) repair. We identified 5 RS-SCID patients without RAG gene mutations, 4 of them withArtemis gene mutations. One patient had a large genomic deletion, but the other 3 patients carried simple missense mutations in conserved amino acid residues in the SNM1 homology domain of the Artemis protein. Extrachromosomal V(D)J recombination assays showed normal and precise signal joint formation, but inefficient coding joint formation in fibroblasts of these patients, which could be complemented by the wild-type Artemis gene. The cells containing the missense mutations in the SNM1 homology domain had the same recombination phenotype as the cells with the large deletion, indicating that these amino acid residues are indispensable for Artemis function. Immunogenotyping and immunophenotyping of bone marrow samples of 2 RS-SCID patients showed the absence of complete V_H-J_H gene rearrangements and consequently a complete B-cell differentiation arrest at the pre–B-cell receptor checkpoint—that is, at the transition from CyIgμ⁻pre-B-I cells to CyIgμ⁺ pre-B-II cells. The completeness of this arrest illustrates the importance of Artemis at this stage of lymphoid differentiation.