Fine mapping of the distal short arm of the human X chromosome using X/Y translocations.

The loci for steroid sulfatase (STS), the deficiency of which causes X-linked ichthyosis, the cell surface antigen 12E7 (MIC2X), and the blood group antigen Xg (Xg) have been mapped to Xp22.3. These loci are of particular interest since they do not appear to undergo X-chromosome inactivation. In an attempt to establish the relative order of STS and MIC2X, fibroblasts from carriers of four different X/Y translocations and an X/10 translocation were obtained and fused with mouse cell lines deficient in hypoxanthine phosphoribosyltransferase. The breakpoints on the X chromosome in these five translocations are in Xp22. Several independent clones from each fusion were isolated in HAT medium. The clones were examined cytogenetically, and in each case at least two independent clones were identified that have an active X/Y or X/10 translocation chromosome in the absence of other X or Y material. These clones were then tested for STS and 12E7 expression. In two of the X/Y translocations, the markers, STS and 12E7, were both absent. In the X/10 and a third X/Y translocation, both markers were retained. In each of three clones containing the fourth X/Y translocation, STS activity was retained but 12E7 antigenicity was lost. Assuming that this is a simple translocation and does not represent a more complex rearrangement, these results suggest that MIC2X is distal to STS.