The mouse WASP gene, the homolog of the gene mutated in Wiskott-Aldrich syndrome, has been isolated and sequenced. the predicted amino acid sequence is 86% identical to the human WASP sequence. A distinct feature of the mouse gene is an expanded polymorphic GGA trinucleotide repeat that codes for polyglycine and varies from 15 to 17 triplets in different Mus musculus strains. The genomic structure of the mouse WASP gene is expressed as an approximately 2.4-kb mRNA in thymus and spleen. Chromosomal mapping in an interspecific M. Musculus/M. spretus backcross placed the Wasp locus near the centromere of the mouse X chromosome, inseparable from Gata1, Tcfe3, and scurfy (sf). This localization makes Wasp a candidate for involvement in scurfy, a T cell-mediated fatal lymphoreticular disease of mice that has previously been proposed as a mouse homolog of Wiskott-Aldrich syndrome. Northern analysis of sf tissue samples indicated the presence of WASP mRNA in liver and skin, presumably as a consequence of lymphocytic infiltration, but non abnormalities in the amount or size of mRNA present.