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 housekeeping two 
genes is within the CpG island and that sequences in the islands have promoter 
function.