Previous studies have shown that approximately 80% of patients with X-linked 
ichthyosis have a total deletion of the steroid sulphatase (STS) locus which 
lies in Xp22.3-Xpter. We show by Southern analysis that a common core of 
sequences are absent in 78.6% of our cases, suggesting that the deletion 
breakpoints may be highly clustered. To characterize the region in more detail a 
long-range physical map of over 3 megabases (Mb) surrounding the STS locus was 
constructed using pulse-field gel electrophoresis. The map enabled the order of 
sequences tel-SI19-GMGXY3-[STS,GMGXY19]-GMGX9-[dic56 ,SIII2]-cen and the 
localization of the deletion breakpoints to be established. In ten cases the 
pulse-field evidence supports the clustering of breakpoints and indicates a 
deletion size of 2 Mb in most patients. Five CpG islands have been positioned 
around the STS locus and may be associated with other loci in the region 
involved in mental retardation and Kallman's syndrome. The map will be 
instrumental in an attempt to isolate and characterize the deletion breakpoints 
and to access other genes located in the region.