Studies of five heterozygous females from three kindreds segregating incontinentia pigmenti indicate that cells expressing the mutation have been eliminated from skin fibroblast cultures and in varying degrees from hematopoietic tissues. Clonal analysis was carried out using G6PD variants and methylation patterns at the HPRT locus. Our results confirm X linkage in these families and suggest that selection against cells expressing mutations that are lethal to males in utero may help ameliorate the deleterious phenotype in carrier females.