Placental mammalian female cells have two X chromosomes. One of these chromosomes is randomly inactivated in each nucleus so that females are functionally mosaic for genes expressed from their X chromosomes. The evolutionary basis for this phenomenon is based on the fact that females would have twice the number of X-linked gene product compared to their male counterpart. This unequal distribution of X-linked genes requires gene dosage compensation. Species that have distinguishable sex chromosomes have evolved different ways to prevent a difference in dosage of the sex chromosome-encoded proteins between the two sexes. In female mammals one X chromosome is transcriptionally inactivated in female somatic cells by a process called X chromosome inactivation (XCI).