X chromosome inactivation during induced differentiation of a female mouse embryonal carcinoma cell line.

P10 is a line of embryonal carcinoma cells with a euploid female karyotype. By making use of variant alleles of X-linked genes and of cytogenetic procedures, we have previously shown that the P10 cells have two genetically active X chromosomes. We show here that P10 cells rapidly differentiate into a cell type resembling extraembryonic endoderm when cultured in the presence of retinoic acid. This differentiation is accompanied by X chromosome inactivation as determined by the appearance of a late-replicating X chromosome. Analysis of the X-linked variant alleles indicated that the P10-derived endoderm did not preferentially inactivate paternally derived X chromosomes. This result is in contrast to the situation in normal extraembryonic endoderm, and suggests that the X inactivation process in differentiating P10 cultures resembles that which occurs in normal embryonic rather than extraembryonic tissues.