Epigenetic alterations and their contribution to carcinogenesis. (A) Loss or gain of different histone modifications can induce aberrant gene expression and promote carcinogenesis in different tumors. The gain of H3K4me3 by HMT and the loss of H3K9ac by HDAC contributes to the expression of genes associated with carcinogenesis. (B) Alterations in the pattern of DNA methylation and the expression of enzymes, such as DNMTs, are found in several tumors. DNA hypomethylation in various regions increases genomic instability and activates proto-oncogenes, whereas DNA hypermethylation favors the silencing of genes, such as tumor suppressors, which contributes to carcinogenesis. (C) Changes in non-coding RNA expression patterns serve an important role in regulating the initiation and progression of various tumors because they can inhibit or increase gene expression through binding to various target genes. Ac, acetylation; DNMT, DNA methyltransferase; H, histone; HDAC, histone deacetylase; K, lysine; HMT, histone methyltransferase; lncRNA, long non-coding RNA; me1, monomethylation; me3, trimethylation; miRNA, microRNA; Pol II, RNA polymerase II; SAM, S-adenosyl methionine; TF, transcription factor.