histone (hĭsˈtōn) [key], any of a class of protein molecules found in the chromosomes of eukaryotic cells. They complex with the DNA (see nucleic acid) and pack the DNA into tight masses of chromatin, which have the structure of coiled coils, much like a tangled telephone cord. The molecules are strongly basic and of relatively small size. They also serve a function in the transcription of DNA. The histones are rich in the amino acids arginine and lysine; the five major subtypes of the class differ from one another chiefly in the relative amounts of these two amino acids. They have been greatly conserved during evolution, histone structures vary little in a wide range of organisms.