||The CATH database is a hierarchical domain classification of protein structures in the Protein Data Bank. Protein structures are classified using a combination of automated and manual procedures. There are four major levels in this hierarchy; Class (secondary structure classification, e.g. mostly alpha), Architecture (classification based on overall shape), Topology (fold family) and Homologous superfamily (protein domains which are thought to share a common ancestor). This colelction is concerned with superfamily classification.