Covalent modifications of histone tails have fundamental roles in chromatin structure and function. One such modification, lysine methylation, has important functions in many biological processes that include heterochromatin formation, X-chromosome inactivation and transcriptional regulation. The DNA within our cells exists in the form of chromatin. The basic building block of chromatin is the nucleosome, a structure consisting of an octamer of four core histone proteins around which 147 base pairs of DNA is wrapped. Core histones are subject to a large number of covalent modifications (PTMs: Post Translational Modifications) including acetylation, methylation, phosphorylation and ubiquitination. Histone methylation occurs on arginine and lysine residues and is catalyzed by enzymes belonging to three distinct families of proteins: the protein arginine specific methyl transferase (PRMT1) family, the SET-domain-containing protein family, and the non-SET-domain proteins DOT1/DOT1L. Unlike acetylation, which generally correlates with transcriptional activation, histone lysine methylation can signal either activation or repression, depending on the sites of methylation. Similar to the discovery that bromodomains can recognize acetylated lysines, studies on histone methylation identified at least three protein motifs (the Chromo-, the Tudor-, and the WD40-repeat domain) that are capable of forming specific interactions with methylated lysine residues of histone.