Hum. Genet. 54, 201-204 (1980) © by Springer-Verlag 1980 Regional Assignment of the Gene Locus for Steroid Sulfatase C. R. Mfiller I, A. Westerveld 2, Beate Migl l, W. Franke 1, and H. H. Ropers ~ Institut ffir Humangenetik und Anthropologie, Albertstr. 11, D-7800 Freiburg, Federal Republic of Germany 2Department of Cell Biology and Genetics, Erasmus Universiteit, Rotterdam, Postbus 1738, The Netherlands Summary. The gene locus for steroid sulfatase, defi- ciency of which causes X-linked ichthyosis, is assigned to Xp 11 ~Xpter by analysis of 24 man-Chinese hamster somatic cell hybrids. High steroid sulfatase activity in a hybrid clone having retained only part of Xq is explain- ed by demonstration of an additional late-replicating human X chromosome. This observation confirms previous evidence for noninactivation of the STS locus. Introduction Deficiency of steroid sulfatase (sterol sulfate sulfo- hydrolase, STS; E.C. 3.1.6.2.) is thought to be the basic enzymatic defect of X-linked ichthyosis (J6bsis et al., 1976). While the enzyme is expressed in normal cul- tured skin fibroblasts, no activity is detectable in fibro- blasts of patients with this dermatologic condition (Shapiro et al., 1978a and b; France, 1978; Kubilus, et al., 1979). X-linked ichthyosis is known to be linked to the Xg gene locus (0 = 0.11). Data are accumulating to support the contention that Xg is not subject to X-chromosome inactivation (Race and Sanger, 1975; Sanger et al., 1977). Evidence was presented by Shapiro et al. (1979) that the neighboring steroid sulfatase gene locus may also escape lyonization. Recent data on the regional chromosomal assign- ment of the STS gene locus obtained by somatic cell genetic methods are conflicting. While Mohandas et al. (1979 a and b) assigned the structural locus to the region Xp22~Xpter, Balazs et al. (1979) mapped it on the long arm between Xq13~Xq24. Offprint requests to: H. H. Ropers In this paper we report on the analysis of a third independent panel of man-Chinese hamster somatic cell hybrids, which assigns the STS gene locus to the short arm of the X chromosome. Materials and Methods Hybrid clones were established as described previously (Wester- veld et al., 1971; de Wit et al., 1977; Pearson et al., 1975). Cells were grown in Dulbecco's medium supplemented with 10% fetal calf serum (Seromed, Munich). Tritium-labeled dehydroepian- drosterone sulfate [7-3H(N)] (24 Ci/mol) was obtained from New England Nuclear (Boston, Mass., USA) and dehydroepiandro- sterone sulfate from E. Merck (Darmstadt). Harvesting of cells and determination of STS activity were performed essentially as described by Hameister et al. (1979), 1 x 10 -4M dehydroepiandrosterone sulfate (= 6 x 104cpm) being used as a substrate. Enzyme activity is expressed as picomoles of dehydroepiandrosterone liberated per milligram of total cell protein per hour. Presence of translocation fragments was ascertained by analysis of the X-borne marker enzymes PGK, GALA, G-6PD, and HPRT; translocation chromosomes involving the short arm of the X were identified by chromosome analysis, supplemented in part by analysis of enzyme markers on the corresponding autosomal fragments (see Table 1). Karyotyping and enzyme analyses were done at the same passage; human chromosomes were identified by quinacrine banding followed by alkaline Giemsa staining. BrdU incorporation studies were done accord- ing to Wolff and Perry (1974). Electrophoresis and staining of the marker enzymes followed published procedures: malate dehydrogenase (E.C. 1.1.1.37.; Harris and Hopkinson, 1976); glutathion reductase (E.C. 1.6.4.2.; Van Someren et al., 1974); phosphoglycerate kinase (E.C. 2.7.2.3.), glucose-6-phosphate dehydrogenase (E.C. 1.1.1. 49.), hypoxanthine-guanine phosphoribosyl transferase (E.C. 2.4.2.8.; Meera Khan, 1971); and c~-galactosidase (E.C. 3.2.1.22.; Grzeschik et al., 1972). 0340-6717/80/0054/0201/$ 01.00