Fig. 1

Methylation of a promoter CpG island (CGI) and transcription of its downstream gene. Open and closed circles Unmethylated and methylated CpG sites, respectively. a In a normal cell, most CpG sites within a promoter CGI are unmethylated. b Methylation of most CpG sites (dense methylation) of the promoter CGI completely blocks transcription. If such methylation occurs in a tumor-suppressor gene, it leads to inactivation of the tumor-suppressor gene