Figure 1. General organization of histone proteins. All five histones consist of a globular domain (indicated as circles) and extended, flexible protein portions. In core histones, the structured domain consists of the histone fold motif and adjacent -helix structures. The length of the individual portions is given in amino acid numbers, the numbers of lysine (K) and arginine (R) residues is indicated for each domain (in parentheses). Due to slight differences within the H1 family, ranging from 193 (H1°) to 220 amino acid residues (H1.3), average numbers are given; the lysine (K) and arginine (R) data refer to the human subtype H1.2. The core histone domain dimensions are based on crystallographic data (Luger et al., 1997) and human histone sequences (Albig and Doenecke, 1997). The N-terminal domains of the core histones contain multiple sites for posttranslational modification by acetylation, methylation or phosphorylation.

Figure 2. Distribution of histone genes within the two clusters on the short arm of human chromosome 6 and on the long arm of chromosome 1. The bars indicate histone genes in their order of arrangement at the 6p21.3–22 and the 1q21 locus. The only known nonhistone gene within the clusters is the hemochromatosis (HFE) gene that is mutated in most cases of hemochromatosis, an iron storage disease. Histone pseudogenes are presented as dotted lines. Mb (megabases) = 10⁶ deoxynucleotide pairs; kb (kilobases) = 10³ deoxynucleotide pairs; cen and ter indicate the orientation (centromeric versus telomeric) of the cluster. The orientation of the cluster on chromosome 1 and the distance between its two subgroups of histone genes are unknown.