Negative selection in early embryogenesis against cells bearing a mutant allele has been described in heterozygote female carriers of X‐linked Fanconi anemia (FANCB, MIM 300514) (Meetei et al., 2004), Bruton agammaglobulinemia (BTK, MIM 300755) (Allen et al., 1994), severe combined immunodeficiency syndrome (IL2RG, MIM 300400) (Puck, Nussbaum, & Conley, 1987), and dyskeratosis congenita (DKCX, MIM 305000) (Devriendt et al., 1997). The gene discussed is FANCB; the disease is dyskeratosis congenita, X-linked.