YAC contig organization and CpG island analysis in Xq28.

One hundred nineteen YACs were assembled into 6 contigs spanning about 7.1 Mb of Xq28. The contigs were formatted with 65 STSs and 136 hybridization probes and were extensive enough to be aligned and oriented by published genetic linkage and somatic cell hybrid panel data. Selected YACs from the entire region were mapped with five rare-cutter restriction enzymes to infer the position of putative CpG islands indicative of gene locations; 48 such sites were identified by the near-coincidence of at least three rare-cutter sites. The analysis defined three subregions of Xq28: 4 Mb of moderate GC and CpG island content from the Xq27 border through the GABRA locus; 1.5 to 2 Mb, extending to the G6PD gene, that is variably and poorly cloned, but contains a high concentration of CpG islands and GC; and about 1.5 Mb between G6PD and the telomere, which is generally low in CpG and GC levels, including a subtelomeric DNA region that shows extensive homology to Yq DNA.