CPg Islands of the X chromosome are gene associated

Unmethylated CpG rich Islands are a feature of vertebrate DNA: they are associated with housekeeping and many tissue specific genes. CpG Islands on the active X chromosome of mammals are also unmethylated. However, Islands on the inactive X chromosome are heavily methylated. We have identified a CpG island in the 5' region of the G6PD gene, and two Islands forty Kb 3' from the G6PD gene, on the human X chromosome. Expression of the G6PD gene is associated with concordant demethylation of all three CpG islands. We have shown that one of the two Islands is in the promoter region of a housekeeping gene, GdX. In this paper we show that the second CpG island is also associated with a gene, P3. The P3 gene has no homology to previously described genes. It is a single copy, 4 kb gene, conserved in evolution, and it has the features of a housekeepin two genes is with in the CpG island and that sequences in the Islands have promoter function.