The basic repeating structural (and functional) unit of chromatin is the nucleosome, which contains nine histone proteins and about 166 base pairs of DNA. The observation by electron microscopists that chromatin appeared similar to beads on a string provided an early clue that nucleosomes exist. Another clue came from chemically cross-linking (i.e., joining) histones in chromatin. This experiment demonstrated that H2A, H2B, H3, and H4 form a discrete protein octamer, which is fully consistent with the presence of a repeating histone-containing unit in the chromatin fiber