The causes and consequences of random and non-random X chromosome inactivation in humans

X chromosome (X) inactivation is a remarkable biological process including the choice and cis-limited inactivation of one X, as well as the stable mai ntenance of this silencing by epigenetic chromatin alterations. The process results in females generally being mosaic for two populations of cells - o ne with each parental X active. In this review, we discuss recent advances in our understanding of how inactivation works, as well as the causes and c linical implications of deviations from random inactivation.