The basic building blocks of chromatin are nucleosomes, each of which is composed of 146 bp of DNA wrapped around a histone octamer that consists of two copies each of H2A, H2B, H3 and H4.

1 Department of Molecular Biology, Princeton University, Princeton, NJ 08544, USA 2 Department of Chemistry, Princeton University, Princeton, NJ 08544, USA Histone post-translational modifications ...