Table 1.

Key Terms

Term	Description
Chromatin	The overall nucleoprotein complex that acts as both a functional and structural scaffold to coordinate access to and control of DNA.
Heterochromatin	Chromatin that is tightly condensed into higher order structures to disallow access to specific genomic regions and the genetic material within.
Euchromatin	Chromatin that is open and uncondensed, enabling access to the genomic region, binding of transcription factors and gene transcription.
Histones	Core proteins that assemble to form an eight protein complex upon which genomic DNA is wound at 147 base pairs to form a single nucleosome. Each octameric histone protein is composed of two H3–H4 dimers bridged together to form a tetramer coordinated with two H2A-H2B dimers.
Histone Modifications	Chemical moieties on histone tails that function as editable epigenetic instructions for directing and coordinating cellular machinery to achieve a particular chromatic state.
DNA Methyltransferases (DNMTs)	Family of enzymes that catalyze the reaction transferring a methyl group from methionine and cytosine to form 5mC.
• DNMT1	Classically considered to be solely responsible for maintenance methylation, ensuring daughter DNA strands maintain the same methylation pattern as the parent strands. DNMT1 has also recently been shown to play a role in de novo methylation [17, 18].
• DNMT3A and DNMT3B	Primarily responsible for so-called de novo methylation, viz. methylation of previously unmethylated cytosine bases. Recent studies have also shown the de novo DNMTs to be important for maintenance methylation of repeat elements [19].
• DNMTL	A closely related homolog to the DNMT proteins that lack methyltransferase activity. DNMTL binds to H3K4 and actively recruits de novo DNMTs to chromatin.
Methylcytosine Binding Proteins (MBDs)	A family of proteins whose putative function is to recognize and bind 5mC in order to translate DNA methylation into functionally silent chromatin [20].
Polycomb Repressive Complexes (PRC1 and PRC2)	Coordinated multiprotein complexes of the Polycomb group (PcG) of proteins that typically act to repress gene expression.
Ten-Eleven Translocation (TET) Enzymes	A family of enzyme that are thought to be involved in active cytosine demethylation by catalyzing the iterative oxidation of 5mC to yield 5-Hydroxymethylcytosine (5hmC), 5-formylcytosine, and 5-carboxylcytosine [21, 22].