The inactivation of all but one of the X chromosomes in cells of female mammals to prevent an abnormal dose of X-linked genes compared with cells of male individuals, which carry a single X chromosome. Hence, in humans, one of the two X chromosomes is normally selected for inactivation and assumes a highly condensed state as a Barr body. X inactivation is controlled by a genetic locus called the X inactivation centre. This includes the XIST gene, which is transcribed as long noncoding RNAs (lncRNAs). These coat the inactivated chromosome, causing it to become condensed as heterochromatin, as in a Barr body, and hence largely inactivated. Some genes in the ‘silenced’ chromosome remain active, particularly in the pseudoautosomal region, a region near one of the telomeres. See also sex chromosome.