Escapees on the X chromosome

In many organisms, differentiation of the sex chromosome complement resulted in the coordinated regulation of genes on whole chromosomes to equalize gene expression between the sexes. In mammals, X inactivation evolved to restore equal expression of X-linked genes in males and females (1). Although X inactivation consists in the general repression of most genes on the X, some genes escape inactivation (reviewed in ref. 2). Recent advances in the Human Genome Project now allow the inactivation status of many X-linked genes to be systematically studied. In this issue of PNAS, Carrel et al. (3) report such a systematic analysis. Their data are so extensive that they are summarized in the paper but are fully accessible only as a file on the World Wide Web (www.pnas.org/supplementary.shtml).