The X chromosome in mammalian somatic cells is subject to unique regulation - usually genes on a single X chromosome are expressed while those on other X chromosomes are inactivated1. The X-locus for steroid sulphatase (STS; EC 184.108.40.206), the microsomal enzyme that catalyses the hydrolysis of various 3β-hydroxysteroid sulphates, is exceptional because it seems to escape inactivation. Evidence for this comes from fibroblast clones in females heterozygous for mutations that result in a severe deficiency of this enzyme in affected males; all clones from these heterozygotes have STS activity, and enzyme-deficient clones that are expected if the locus were subject to inactivation2, have not been found3. Further evidence that the STS locus escapes inactivation is that the human inactive X chromosome contributes STS activity to mouse-human hybrid cells4. On the basis of these hybrid studies5,6 the STS locus has been mapped to the distal half of the short arm (p22-pter) of the human X chromosome. Although the STS locus on both X chromosomes in human female cells is expressed, quantitative measurements of STS activity in males and females do not accurately reflect the sex differences in number of X chromosomes7-13 (Table 1). The ratio of mean values for normal females to that of normal males is greater than 1:1 but less than the ratio of 2:1 expected if STS loci on all X chromosomes were equally expressed. The incomplete dosage effect suggests that the STS locus on the inactive X chromosome might not be fully expressed. To test this hypothesis, we examined two heterozygotes for X-linked STS deficiency who were also heterozygous for the common electrophoretic variants of glucose-6-phosphate dehydrogenase (G6PD A and B). Studies of fibroblast clones from these females provide evidence, presented here, for differential expression of STS loci on the active and inactive X chromosome.
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