each fold are given. For example, the "globin-like" fold is described as core: 6 helices; folded leaf, partly opened. The fold to which a domain belongs is determined by inspection, rather than by software. The levels of SCOP version 1.75 are as follows. Class: Types of folds, e.g., beta sheets. Fold: The different shapes of domains within a class. Superfamily: The domains in a fold are grouped into superfamilies, which have at least a distant common ancestor. Family: The domains in a superfamily are grouped into families, which have a more recent common ancestor. Protein domain: The domains in families are grouped into protein domains, which are essentially the same protein. Species: The domains in "protein domains" are grouped according to species. Domain: part of a protein. For simple proteins, it can be the entire protein. Classes The broadest groups on SCOP version 1.75 are the protein fold classes. These classes group structures with similar secondary structure composition, but different overall tertiary structures and evolutionarily origins. This is the top level "root" of the SCOP hierarchical classification. All alpha proteins [46456] (284): Domains consisting of α-helices All beta proteins [48724] (174): Domains consisting of β-sheets Alpha and beta proteins (a/b) [51349] (147): Mainly parallel beta sheets (beta-alpha-beta units) Alpha and beta proteins (a+b) [53931] (376): Mainly antiparallel beta sheets (segregated alpha and beta regions) Multi-domain proteins (alpha and beta) [56572] (66): Folds consisting of two or more domains belonging to different classes membrane and cell surface proteins and peptides [56835] (58): Does not include proteins in the immune system Small proteins [56992] (90): Usually dominated by metal ligand, cofactor, and/or disulfide bridges coiled-coil proteins [57942] (7): Not a true class Low resolution protein structures [58117] (26): Peptides and fragments. Not a true class Peptides [58231] (121): peptides and fragments. Not a true class. Designed proteins [58788] (44): Experimental structures of proteins with essentially non-natural sequences. Not a true class The number in brackets, called a "sunid", is a SCOP unique integer identifier for each node in the SCOP hierarchy. The number in parentheses indicates how many elements are in each category. For example, there are 284 folds in the "All alpha proteins" class. Each member of the hierarchy is a link to the next level of the hierarchy. Folds Each class contains a number of distinct folds. This classification level indicates similar tertiary structure, but not necessarily evolutionary relatedness. For