Evidence for the inactivation of an X chromosome early in the development of the human female.

Studies of somatic tissues and cultured cells, including fibroblast clones, from human embryos heterozygous for the electrophoretic variants of glucose-6-phosphate dehydrogenase confirm that one X chromosome is inactivated very early in embryonic development and indicate that X inactivation has occurred in the majority of cells from a variety of tissues at least by 5 weeks from conception.