Xist RNA exhibits a banded localization on the inactive X chromosome and is excluded from autosomal material in cis

The propagation of X chromosome inactivation is thought to be mediated by t he cis-limited spreading of the nonprotein coding Xist transcript. In this report we have investigated the localization of Xist RNA on rodent metaphas e chromosomes. We show that Xist RNA exhibits a banded pattern on the inact ive X and is excluded from regions of constitutive heterochromatin. The ban ding pattern suggests a preferential association with gene-rich, G-light re gions. Analysis of X:autosome rearrangements revealed that restricted propa gation of X inactivation into cis-linked autosomal material is reflected by a corresponding limited spread of Xist RNA. We discuss these results in th e context of models for the function of Xist RNA in the propagation of X in activation.