There is preferential inactivation of the paternal X chromosome in extraembryonic tissues and in all cells in the early embryo (Takagi
N and Sasaki M 1975; West JD et al 1977; Mak W et al
2004), resulting from preferential expression of the paternal Xist (Kay GF et al
Paternally biased X inactivation has been reported in the mouse neonatal brain (Wang X et
Xist function is regulated by the antisense transcript Tsix (Lee JT et al 1999).