Mammalian X-chromosome action: inactivation limited in spread and region of origin.

In its simplest form the hypothesis of the single-active-X chromosome does not explain variegated-type position effects in the mouse. Inactivity appears not to involve one entire X chromosome; furthermore, even those parts of the chromosome that can change to an inactive state spread inactivation not to the entire attached piece of autosome, but along a gradient to limited distances.