Mutations at the W locus in the mouse have pleiotropic effects on embryonic development and hematopoiesis. The characteristic phenotype of mutants at this locus, which includes white coat color, sterility, and anemia, can be attributed to the failure of stem cell populations to migrate and/or proliferate effectively during development. Mapping experiments suggest that the c-kit proto-oncogene, which encodes a putative tyrosine kinase receptor, is a candidate for the W locus. We show here that the c-kit gene is disrupted in two spontaneous mutant W alleles, W44 and Wx. Genomic DNA that encodes amino acids 240 to 342 of the c-kit polypeptide is disrupted in W44; the region encoding amino acids 342 to 791 is disrupted in Wx. W44 homozygotes exhibit a marked reduction in levels of c-kit mRNA. These results strongly support the identification of c-kit as the gene product of the W locus.