IN the past, the structure of chromatin has remained obscure despite the efforts of many laboratories and seemed far from a simple solution1. It has been known only that chromatin contains a repeating substructure2,3. Recently, however, Kornberg proposed4 that chromatin structure is based on a repeating subunit of 200 base pairs of DNA and two of each of the histones (with the exception of F1 which occurs only once per subunit). Furthermore, he suggested that in chromatin these subunits form a flexibly jointed chain.