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The retinoblastoma gene (Rb) is one of the best characterized tumor suppressor genes, and its inactivation is associated with a number of cancers. Previous studies have shown, by restriction enzyme analysis, that the promoter region of the Rb gene is methylated in a significant proportion of primary retinoblastoma tumors. We now report the first detailed methylation sequence analysis of the CpG island spanning the retinoblastoma promoter from hypermethylated retinoblastoma tumors. Our results show methylation is not confined to a specific CpG site, as detected by restriction enzyme studies, but extends to essentially all 27 CpG dinucleotides spanning the retinoblastoma CpG island, including the core promoter. The methylation pattern from each tumor DNA sample is different, ranging from densely to sparsely methylated profiles. Single CpG sites, in particular the E2F transcription factor binding site, as well as …