Localization of the X inactivation centre on the human X chromosome in Xq13

X-CHROMOSOME inactivation results in the strictly cis-limited inactivation of many but not all genes on one of the two X chromosomes during early development in somatic cells of mammalian females1. One feature of virtually all models of X inactivation is the existence of an X-inactivation centre (XIC) required in cis for inactivation to occur2–5. This concept predicts that all structurally abnormal X chromosomes capable of being inactivated have in common a defineable region of the X chromosome6–8. Here we report an analysis of several such rearranged human X chromosomes and define a minimal region of overlap. The results are consistent with models invoking a single XIC and provide a molecular foothold for cloning and analysing theXIC region. One of the markers that defines this region is theXIST gene9, which is expressed specifically from inactive, but not active, X chromosomes. The localization of the XIST gene to theXIC region on the human X chromosome implicates XIST in some aspect of X inactivation.