Table 1.

Introduction to epigenetic modifications in mammals.

epigenetic modification	modification enzymes	effect on genome function
CpG DNA methylation	DNA methyltransferases (DNMT) 1, 3a and 3b. Base excision/DNA repair proteins or simply failure to remethylate at DNA replication are thought to facilitate demethylation	presence at promoters associated with gene silencing. Presence in gene bodies associated with gene activity
non-CpG DNA methylation	some evidence points to the known DNA methyltransferases	currently unclear, though its presence at genes is usually associated with gene activity
histone acetylation	several histone acetyl transferases (HATs) and histone deacetylases (HDACs)	increases protein access to DNA for transcription or genome-wide reprogramming e.g. sperm protamination
histone methylation	several modification-specific histone methyl transferases and histone demethylases	methylation of some amino acid residues associated with transcriptional repression, while others with activation
histone variants e.g. H2A.Z, CENP-A, H2AX		various specialized functions including centromere function, DNA repair and gene regulation
small non-coding RNAs, e.g. miRNAs, piRNAs	biogenesis and function requires enzymes such as RNA polymerase II, DICER and ARGONAUTE	various effects such as transcriptional repression and activation, translational repression
long non-coding RNAs	biogenesis of most found so far involves RNA polymerase II	known to regulate large-scale transcriptional repression in genomic imprinting. Recent discovery of abundance throughout the genome suggests a high variety of functions