CATH is a classification of protein structures downloaded from the Protein Data Bank. It groups protein domains into superfamilies when there is sufficient evidence they have diverged from a common ancestor. CATH has participated in several European projects and Networks of Excellence to provide data, tools, and interoperability for structural and functional annotation of genome sequences. It also participated in the NIH-funded structural genomics (PSI) initiative, providing information on fold libraries, predicted structures and functions to facilitate target selection for structure determination.