Editing Promoter (genetics) (section)

===Promoter CpG hyper/hypo-methylation in cancer===

Generally, in progression to cancer, hundreds of genes are [[Regulation of transcription in cancer#Transcription silencing/activation in cancers|silenced or activated]].  Although silencing of some genes in cancers occurs by mutation, a large proportion of carcinogenic gene silencing is a result of altered DNA methylation (see [[DNA methylation in cancer]]).  DNA methylation causing silencing in cancer typically occurs at multiple [[CpG site]]s in the [[CpG site#CpG island|CpG island]]s that are present in the promoters of protein coding genes.

Altered expressions of [[microRNA]]s also silence or activate many genes in progression to cancer (see [[MicroRNA#cancer|microRNAs in cancer]]).  Altered microRNA expression occurs through [[Regulation of transcription in cancer#Transcription silencing/activation in cancers|hyper/hypo-methylation]] of [[CpG site]]s in [[CpG site#CpG island|CpG island]]s in promoters controlling transcription of the [[microRNA]]s.

Silencing of DNA repair genes through methylation of CpG islands in their promoters appears to be especially important in progression to cancer (see [[DNA methylation in cancer#Likely role of hypermethylation of DNA repair genes in cancer|methylation of DNA repair genes in cancer]]).