sentences:  High expression of a microRNA causes repression of its target genes (see microRNA silencing of genes).In 66% to 90% of colon cancers, miR-21 was over-expressed, and generally the measured level of hMSH2 was decreased (and hMSH6 is unstable without hMSH2).The other microRNA, miR-155, is regulated both by epigenetic methylation of the CpG islands in its promoter region and by epigenetic acetylation of histones H2A and H3 at the miR-155 promoter (where acetylation increases transcription).