The Structural Classification of Proteins (SCOP) database represents a manual classification of protein structural domains. The classification is based on the similarities of structural features and amino acid sequences, as well as evolutionary relationships. As a result, proteins with similar structures bu distant sequences are placed in different superfamilies, whereas proteins with structural similarities and sequence homology are classified as members of protein families. SCOP is based on the classification of structural domains but not the entire proteins, wheres the Protein Data Bank serves as a source of structures. The levels of SCOP classification comprise class, fold, superfamily, family, protein domain, species, and domain.