Variation in regulation of steroid sulphatase locus in mammals

Inactivation (lyonization) of one of the two copies of X-linked genes occurs in female mammals, thereby reducing the number of active copies to that of the male. It has been suggested that genes subject to lyonization would be expected to be preserved as a linkage group during mammalian evolution. A short region of the human X chromosome containing several genes, including that necessary for the expression of steroid sulphatase (STS), is exceptional in that it apparently escapes X-inactivation. As it is not apparent why the linkage of genes not subject to X-inactivation should be conserved, we have examined the expression of the STS gene in mice (it has been shown recently that this gene is X-linked). Enzyme levels were determined in normal males and females and in the progeny of crosses in which the sex reversing factor, Sxr, was segregating to produce XX males. We report here that in contrast to the situation in humans, the STS gene in mice is subject to the normal pattern of X-inactivation.