Histones are the major structural proteins around which more than 2 m of DNA in every eukaryotic cell is organized. These proteins are considered to be small molecular weight proteins composed of a very high proportion of positively charged amino acids like lysine and arginine. This complex of histone protein, nonhistone protein, and DNA is often referred to as chromatin, the fundamental unit of which is referred to as the nucleosome. The nucleosome consists of a complex of approximately 150 bp of DNA and a histone octamer. Each histone octamer is comprised of a pair of histones including H2A, H2B, H3, and H4 (Fig. 1). Neighboring nucleosomes are linked together by DNA bound to the linker histone (H1). This complex assembly of protein and DNA provides an important organizational structure that helps the cell maintain control over transcription.