In contrast, the epiblast region of carrier female embryos was composed of a mosaic of small clusters of Atrx-positive cells (in which the Atrx null allele on Xm had been inactivated) and Atrx-negative cells (in which the Atrx WT allele on Xp had been inactivated), indicating that the Atrx gene was subject to normal random X-inactivation in the epiblast.