but a directed acyclic graph network connecting protein superfamilies representing structural and evolutionary relationships such as circular permutations, domain fusion and domain decay. Consequently, domains are not separated by strict fixed boundaries, but rather are defined by their relationships to the most similar other structures. The prototype was used for the development of the SCOP version 2 database. The SCOP version 2, release January 2020, contains 5134 families and 2485 superfamilies compared to 3902 families and 1962 superfamilies in SCOP 1.75. The classification levels organise more than 41 000 non-redundant domains that represent more than 504 000 protein structures. The Evolutionary Classification of Protein Domains (ECOD) database released in 2014 is a similar to SCOPe expansion of SCOP version 1.75. Unlike the compatible SCOPe, it renames the class-fold-superfamily-family hierarchy into an architecture-X-homology-topology-family (A-XHTF) grouping, with the last level mostly defined by Pfam and supplemented by HHsearch clustering for uncategorized sequences. ECOD has the best PDB coverage of all three successors: it covers every PDB structure, and is updated biweekly. The direct mapping to Pfam has proven useful to Pfam curators who use the homology-level category to supplement their "clan" grouping. See also Structural alignment CATH FSSP SUPERFAMILY Pfam References External links Structural Classification of Proteins (SCOP 2) - Manual classification of representative domains, regularly updated by the SCOP authors Structural Classification of Proteins (SCOP 1.75) - Legacy SCOP 1.75 site, no longer updated Structural Classification of Proteins extended (SCOPe) - The more automated successor of SCOP version 1.75 Evolutionary Classification of Protein Domains (ECOD) - Evolutionary classification based on SCOP version 1.75 and Pfam Structural Classification of Proteins 2 (SCOP2 prototype) - Legacy site of the SCOP 2 prototype, no longer updated SUPERFAMILY - Library of HMMs representing SCOP superfamilies and database of (superfamily and family) annotations for all completely sequenced organisms Protein Structure Classification - a book chapter that discusses different protein classifications in detail. Biological databases Medical Research Council (United Kingdom) Protein structure Protein folds Protein classification Protein superfamilies Science and technology in Cambridgeshire