Gene expression patterns in cells can be controlled epigenetically through the modification of DNA methylation, by covalently modifying the histone proteins or through the use of microRNAs and RNA interference pathways.  Direct methylation of DNA occurs through the action of DNA Methyltrasferases (DNMT).  DNMT1 is the most abundant DNMT and acts to methylate DNA at CpG dinucleotides.  Histone acetylation is general associated with increased transcriptional activation while methylation levels may either increase or decrease transcription.  MicroRNA act as post-transcriptional regulators of gene expression which specifically bind the mRNA of target genes.