English: Nucleosomes consist of four pairs of histone proteins in a tightly assembled core region plus up to 30% of each histone remaining in a loosely organized tail (only one tail of each pair is shown). DNA is wrapped around the histone core proteins in chromosomes. The pairs of histones, H2A, H2B, H3 and H4, each have lysines (K) or arginines (R) in their tails, some of which are subject to post-translational modifications. If a nucleosome is associated with a DNA region that is to have transcriptional repression the post-translational modifications usually consist of methylations {Me}. The lysines (K) and arginines (R) are designated with a number showing their position as, for instance, (R2) indicating arginine as the 2nd amino acid from the amino (N) end of the tail in the histone protein. The particular methylations {Me} shown are those that occur on nucleosomes close to, or at, some DNA regions undergoing transcriptional repression of the DNA wrapped around the nucleosome.