Histones are a class of DNA-binding proteins that includes five types of protein. Their properties are outlined in Table 28.1. All histones are small, very basic proteins rich in lysine and arginine. Some have been remarkably well conserved in amino acid sequence throughout evolution. Histone H4, for example, shows only two substitutions between humans and peas and only eight substitutions between humans and yeast. The histones are the basic building blocks of chromatin structure. The nucleoids of prokaryotic cells also have proteins associated with DNA, but these proteins are quite different from the histones and do not seem to form a comparable chromatin structure. Thus, a histone-containing chromatin structure is a uniquely eukaryotic feature. In all kinds of eukaryotic nuclei, the histones are present in an equal weight amount with DNA, and histones H2A, H2B, H3, and H4 are always found in equimolar quantities.