In eukaryotes, DNA is wrapped around histone octamers to form the basic unit of chromatin structure. The octamer is composed of histones H2A, H2B, H3 and H4 and it associates with approximately 200 base pairs of DNA to form the nucleosome. The association of DNA with histones results in dense packing of chromatin, which restricts proteins involved in gene transcription from binding to DNA. Histone H1 is required for the condensation of nucleosome chains into higher order structures. Phosporylation of histone H1 is thought be involved in this process, although the exact nature of this role has yet to be elucidated. Evidence suggests that histone H1 is a part of a general repressor mechanism for stable repression of transcription, but it can also activate transcription of specific genes.