|  | Class a: All alpha proteins [46456] (202 folds) | 
|  | Fold a.22: Histone-fold [47112] (1 superfamily) core: 3 helices; long middle helix is flanked at each end with shorter ones | 
|  | Superfamily a.22.1: Histone-fold [47113] (3 families)  | 
|  | Family a.22.1.1: Nucleosome core histones [47114] (4 proteins) form octamers composed of two copies of each of the four histones | 
|  | Protein Histone H2B [47119] (3 species) | 
|  | African clawed frog (Xenopus laevis) [TaxId:8355] [47121] (20 PDB entries) | 
|  | Baker's yeast (Saccharomyces cerevisiae), H2B.2 [TaxId:4932] [68985] (1 PDB entry) | 
|  | Chicken (Gallus gallus), erythrocytes [TaxId:9031] [47120] (4 PDB entries) | 
Timeline for Protein Histone H2B from a.22.1.1: Nucleosome core histones: