Percentage of progenitor cells from bone marrow or fetal liver of heterozygous female expressing normal (X⁺) or mutated (X^–) allele. Bone marrow and fetal liver cells from heterozygous females were seeded into methylcellulose in the presence or absence of G418. Only the cells expressing the X-chromosome carrying the mutated WASP gene are able to grow in the presence of G418. Fetal liver cells from heterozygous females were also injected intravenously into irradiated recipient mice, and at 24 hours or 3 months after the graft, bone marrow cells were harvested and cultured in methylcellulose in the presence or absence of G418. At 7 days later, the number of colonies were scored. Results are expressed as the percentage of clonogenic progenitors expressing the normal (X⁺) or the mutated (X^–) allele. This proportion was compared (by Wilcoxon test and χ² test) to the proportion 50:50 that is observed when random X-chromosome inactivation occurs. As this skewed X inactivation is observed 24 hours after transplantation, this indicates that hematopoietic progenitors expressing WASP have a selective advantage over the WASP-deficient progenitors in homing ability. Error bars indicate SD.