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The IL2RG gene encoding the gamma chain of the lymphocyte receptor for IL-2 lies in human Xq13.1 and is mutated in males with X-linked severe combined immunodeficiency (SCID). In a large Canadian pedigree genetic linkage studies demonstrated that the proband's grandmother was the source of an X-linked SCID mutation. However, her T cells did not show the expected skewed X chromosome inactivation pattern of female carriers of SCID, despite her having one affected son and two carrier daughters with skewed X inactivation. Single strand conformation polymorphism analysis of IL2RG in the affected proband was abnormal in exon 5; sequencing revealed a nine nucleotide in-frame duplication insertion. The three duplicated amino acids included the first tryptophan of the "WSXWS" motif found in all members of the cytokine receptor gene superfamily. Mutation detection in the pedigree confirmed that the founder grandmother's somatic cells had only normal IL2RG, and further showed that the SCID-associated X chromosome haplotype was inherited by three daughters, one with a wild type IL2RG gene and two others with the insertional mutation. Female germ line mosaicism is unusual, but its presence in this X-linked SCID family emphasizes the limitations of genetic diagnosis by linkage as compared with direct mutation analysis.