Histone post-translational modifications were first identified and listed as having a potential regulatory role on the synthesis of RNA in 1964.   Since then, over several decades, chromatin theory has evolved.  Chromatin subunit models as well as the notion of the nucleosome were established in 1973 and 1974, respectively. Richmond and his research group has been able to elucidate the crystal structure of the histone octamer with DNA wrapped up around it at a resolution of 7 Å in 1984.  The structure of the octameric core complex was revisited seven years later and a resolution of 3.1 Å was elucidated for its crystal at a high salt concentration.  Though sequence similarity is low between the core histones, each of the four have a repeated element consisting of a helix-loop-helix called the histone fold motif.  Furthermore, the details of protein-protein and protein-DNA interactions were fine-tuned by X-ray crystallography studies at 2.8 and 1.9 Å, respectively, in the 2000s.