Escape from X inactivation in human and mouse

Genes that escape X inactivation have been recently found in human and in mouse. Although many of these genes have homologues on the Y chromosome that may compensate for expression from both X alleles in females, some have no Y homologues, and this presumably results in dosage differences between the sexes. Comparisons between human and mouse have revealed that the X-inactivation status of some genes differs significantly between the two species, suggesting continuous evolutionary changes in the sex chromosomes. Questions about the mechanisms of 'escape' are relevant to the understanding of gene regulation by X inactivation.