X-chromosome inactivation and clonal development with further subclone evolution. (A) Clonal populations can acquire multiple genetic abnormalities; each can be used to distinguish subpopulations which remain clonal. Round white Xp, nonclonal cell with active paternal X chromosome; round gray Xp, clonal cell with active paternal X chromosome; oval dark gray Xp⁺, clonal cell with active paternal X chromosome and new somatic mutation. (B) X-chromosome inactivation. Early in development, both X chromosomes are expressed. Before hematopoietic lineage differentiation, either X chromosome is inactivated. This random choice of inactivated X chromosome is retained through subsequent mitoses. Oval XpXm indicates early embryonic cells expressing both maternal and paternal X chromosomes; round Xp or Xm, nonclonal cells expressing a single X chromosome.