English: The histone octamer is formed from two subunits each of the four core histones. The four H3 and H4 subunits form a tightly packed tetramer that associates with two H2A/H2B dimers to form each octamer. About 147 bp of DNA is wrapped around the octamer to form a nucleosome. Nucleosomes can be arrayed in the loosely packed beads-on-a-string form of chromatin, but are generally more tightly packaged into the 30-nm fiber. Fiber formation requires histone tails and additional proteins, neither of which is shown here.^[1]