X chromosome expression in normal and molar pregnancy.

In the female mouse embryo the maternal X chromosome is selectively expressed in derivatives of the trophectoderm and the primitive endoderm. In the primitive ectoderm derivatives the expression of the maternally or paternally derived X chromosome is random. Although this selective expression is not an essential condition for the trophectoderm and the primitive endoderm to develop, the purpose of this selective expression is unknown. It is postulated that in human reproductive biology a key to the purpose of this selective expression may be found by the hydatidiform mole, due to its androgenic origin.