We report the results of studies on the characterization of the mutation 
associated with marked unbalanced expression of the mutant X chromosome in a 
karyotypically normal girl with Hunter disease (mucopolysaccharidosis type II). 
Southern analysis of DNA extracted from somatic cell hybrids containing only the 
mutant X chromosome showed deletion of the Xq27.3-q28 loci: DXS297 (VK23AC), 
DXS293 (VK16), FRAXA (pfxa3), DXS296 (VK21A), and the 3' end of the 
iduronatesulfatase (IDS) gene. The flanking loci--DXS52 (St14-1), DXS304 (U6.2), 
and DXS369 (RN1)--were intact. On the basis of these results, we concluded that 
the mutation was a simple deletion extending a maximum of 3-5 cM to the 
centromeric side of the IDS gene. Both Southern analysis of DNA from somatic 
cell hybrids, using short segments of IDS cDNA, and PCR of reverse-transcribed 
RNA from cultured skin fibroblasts indicated that the telomeric terminus of the 
deletion was localized to a region near the middle of the coding sequences of 
the gene.