Hierarchical classification of protein domain structures that clusters proteins at four major levels, Class, Architecture, Topology, and Homologous superfamily. ~26 million protein domains classified into ~2,740 superfamilies.

Structural Classification of Proteins, database of experimentally determined protein structures, divided into domains. Alpha proteins, beta, alpha and beta (a/b, a+b), multi-domain, membrane and cell surface proteins and peptide folds, small proteins, coiled-coil, low resolution, peptides, designed protein folds.