X chromosome inactivation in mammals. Inactivation of one X chromosome in each cell of the female mammal begins at the XIC locus, which encodes XIST (a non-protein-coding RNA), and also requires the histone variant macroH2A and other chromatin regulatory factors. XIST coats the X chromosome, starting at the nucleation site and then spreading in both directions until the entire chromosome is coated, resulting in repression of nearly all the genes on the chromosome.