Histone proteins H3, H4, H2A, and H2B function as building blocks to package eukaryotic DNA into repeating nucleosome units that are folded in higher order chromatin fibers. The nucleosome is composed of an octamer containing a H3/H4 tetramer and two H2A/H2B dimers, surrounded by approximately 146 base pairs of DNA. A diverse and elaborate array of post-translational modifications including acetylation, phosphorylation, methylation, ubiquitination, and ADP-ribosylation occurs on the N-terminal tail domains of histones. Methylation of position-specific lysine residues in histone N termini is a central modification for regulating epigenetic transitions in chromatin. Each methylatable lysine residue can exist in a mono, di, or tri methylated state. Arginine resdiues can also by mono or di methylated.