Any of a group of water-soluble proteins found in association with the DNA of plant and animal chromosomes to form chromatin. They contain a large proportion of the basic (positively charged) amino acids lysine, arginine, and histidine, and hence bind tightly to the negatively charged DNA. There are five classes of histones—H1, H2A, H2B, H3, and H4—involved in the condensation and coiling of chromosomes. The DNA winds around clusters of histones forming the spool-like nucleosomes, the basic units of DNA packaging. Each nucleosome comprises a pair of histones from each of classes H2A to H4 plus a single H1 histone. Chemical modification of histones is a key aspect of suppressing or activating gene activity (see chromatin remodelling). Histones do not occur in vertebrate sperm cells (see protamine) or in bacteria, although a very similar protein has been found in the genome of the archaean Thermoplasma. See also epigenome; gene imprinting.