Somatic selection after X-chromosome inactivation. (A) Early embryonic cells express both X chromosomes and undergo X-chromosome inactivation. (B) After inactivation, embryonic cells express one or the other X chromosome. Thus, in a disease like Wiskott-Aldrich syndrome, a heterozygous female can express both the normal and mutant alleles, albeit in different cells. (C) The embryonic cells undergo proliferation. (D) Cellular selection against the cells bearing the active X chromosome with the mutant allele results in elimination of that population of cells, resulting in (E) an apparently clonal (“pseudoclonal”) population.