The structural and functional subunits of chromatin are nucleosome cores. In a nucleosome core 145 bp of DNA are coiled around the outer surface of an octamer of histone proteins which consists of a tetramer of 2(H3�H4) and two H2A�H2B dimers (1 ). DNA extending from the nucleosome core to the next nucleosome is called linker DNA. It varies in length from about 20 to 90 bp in different organisms or tissues or between individual nucleosomes. Histone H1 may be associated with linker DNA at the site where the DNA leaves the nucleosome. While core histones are well conserved and present in all eukaryotic organisms, H1 is most variable and may even be missing in some organisms such as yeast Saccharomyces cerevisiae . Nucleosomes are built from many different DNA sequences and may contain histone variants (subtypes) and modified histones (e.g., acetylated) which can affect their structural and dynamic properties (reviewed in ref. 2 ).