"Sexually dimorphic organisms employ the services of epigenetics - heritable changes in gene expression that are independent of DNA sequence - to balance genetic differences between the two sexes. A superb model of his relationship, X-chromosome inactivation, has evolved uniquely in mammals to ensure equal gene dosage between females, who have two X chromosomes, and males who have only one X. This precise pathway results in the silencing of the majority of genes on one X chromosome early in female development. This outcome requires a female cell to undergo a highly orchestrated set of events when it differentiates. A cell must count the X chromosomes, choose one X to inactivate (usually in a random manner), initiate and propagate chromosome-wide silencing, and finally maintain this inactive state throughout subsequent cell divisions (1). Shortly after the discovery of X inactivation by Mary Lyon in 1961, geneticists hypothesized that cis-acting factors (acting on the same chromosome) encoded by the X must be important in this process. Likewise, trans-acting factors (acting on different chromosomes) encoded by chromosomes other than the X or Y were presumed to be equally important )2)." ........Percec, I. and M. S. Bartolomei. 2002. Do X chromosomes set boundaries? Science 295(5553): 287-8.