The XIST gene at Xq13.2 specifies transcripts of a long non coding RNA (lncRNA, there is extensive alternative splicing) that is essential for the stability of the repressed X chromosome by coating the DNA beneath the repressing histones in the (Barr body). This is essential for dosage compensation which in mammals is achieved by X inactivation, although other eukaryotes can have other mechanisms for dosage compensation of their sex chromosomes. XIST is a key component of the the X inactivation center (XIC at Xq13.2), where X inactivation starts. It is expressed after 1000 odd embryonic divisions. Mutations in the XIC affecting XIST expression occur at the XIST promoter cause familial skewed X inactivation and elsewhere to cause a syndrome of multiple congenital malformations, severe mental retardation associated with ring X chromosomes. Interactions occur with BRCA1 suggesting wider activity than its key sexual role.