TRANSGENIC MICE CARRYING AN XIST-CONTAINING YAC

The initiation of X-chromosome inactivation in female mammals is contr olled by a key locus, the X-inactivation centre (Xic). The Xist gene, which maps to the candidate region for Xic and is expressed exclusivel y from the inactive X chromosome, is thought to be an essential compon ent of the Xic, To test whether sequences spanning several hundred kil obases and including Xist from the Xic region are capable of initiatin g inactivation, we have created a series of transgenic mice using a 46 0 kb yeast artificial chromosome (YAC). Analysis in these mice of the expression of Xist, of a LacZ reporter gene and of two genes in the re gion that are normally silent on the inactive X chromosome, suggests t hat essential sequences for Xist expression and X-inactivation may be absent in these transgenic animals.