Expression of the maternally derived X chromosome in the mural trophoblast of the mouse

Only the maternally derived allelic form of the X-chromosome-linked enzme phosphoglycerate kinase (PGK-1) is observed in the mural trophoblast of heterozygous female progeny in F1 and backcross matings. We have demonstrated that this expression of the maternally derived PGK-1 is not a result of maternal tissue contamination nor of selection of cells expressing the maternal X chromosome (Xm). Our results suggest that the expression of Xm in mural trophoblast is a consequence of nonrandom X-chromosome inactivation in trophectoderm cells.