DNA sequences of the X-chromosome-linked hypoxanthine phos- phoribosyltransferase (HPRT) and glucose 6-phosphate dehydrogenase (G6PD) genes have revealed the presence of clusters of CpG dinucleotides1-3, raising the possibility that such clusters are involved in the control of expression of these genes, which are expressed in all tissues. Although CpG clusters are not exclusive features of the X chromosome4-6, the analysis of X-linked genes provides the means to determine whether CpG clusters are control elements; one of the two homologous X loci in female mammals is not expressed7, so that active and inactive versions of the gene can be compared. In fact, it has been shown that these CpG clusters are undermethylated when the gene is active and extensively methylated when the gene is inactive2,8. In addition to hypomethylation, chromatin hypersensitivity to endonuclease digestion is a known hallmark of regulatory sequences in eukaryotic genes9. We report here that the CpG clusters of the active hprt and g6pd genes are not only undermethylated, but also hypersensitive to MspI, DNase I and S1 nuclease, further supporting the suggestion that they are involved in the control of expression of these genes.
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