During development, in female cells, X-chromosome inactivation occurs randomly for one of the two X chromosomes, which equalizes the dosage of gene products from the X chromosome between females (XX) and males (XY). Each X chromosome has an equal probability of being silenced. Once established, inactivation of the X chromosome is stable, and perpetuates throughout subsequent division of the daughter cells. Through this process, each female is a mosaic of cells, composed of cells where either the maternally or paternally inherited X is expressed, and the other inherited X is silenced.