Stability of X chromosome differentiation in mouse embryos. Reversal may not be responsible for the extreme X-inactivation mosaicism in extraembryonic membranes.

By means of a double labeling method with H3-thymidine and 5-bromodeoxyuridine, it was found that the X chromosome showed no sign of change from an allocyclic to an isocyclic state, or vice vers in 6.5- and 7.5-day mouse embryos. Thus, reversal of allocycly may not account for the predominance of cells with the paternally derived X chromosome inactive in the yolk sac and the chorion of the mouse embryo.