Reports on inactivation of x-chromosomes in all female mammals to ensure that X-linked gene dosage is equal in males and females. Study by Edith Heard and colleagues which shows that X-chromosome inactivation occurs earlier than previously indicated; Inactivation of the paternal X (Xp) chromosome; Indication that the paternal X chromosome is rapidly inactivated at the four-cell stage, but is then reversed during inner cell mass growth; Highly labile state of Xp during early development.