CORUM is a manually curated database of experimentally characterized protein complexes from mammalian organisms, mainly human (64%), mouse (16%), and rat (12%). The CORUM 2.0 release includes 2837 protein complexes, representing approximately 16% of human protein-coding genes. Each complex is described by name, subunit composition, function, and literature references. Recent developments include mapping to Gene Ontology terms and Entrez Gene identifiers, as well as a 'Phylogenetic Conservation' tool for predicting orthologous subunits in different organisms. CORUM is freely accessible at http://mips.helmholtz-muenchen.de/genre/proj/corum/index.html. The database provides a comprehensive dataset of mammalian protein complexes, with a focus on their subunit composition, function, and disease associations. It includes a 'Core Set' of 2084 distinct complexes to avoid redundancy. CORUM is used for various bioinformatics analyses, including tissue-specific expression, functional interpretation of high-throughput data, and predicting protein interactions. It also contributes to web-based applications like the DICS database and COFECO tool. The CORUM dataset has been used to analyze the composition and reuse of protein complex subunits, showing that most proteins are part of only one complex, while some are reused in multiple complexes. The 'Phylogenetic Conservation' tool allows prediction of orthologous subunits in different organisms, based on sequence similarity data from the SIMAP database. This tool helps in understanding the evolutionary conservation of protein complexes. CORUM is the only resource that includes functional annotation of protein complexes using the FunCat scheme. It also provides mappings from FunCat to Gene Ontology (GO) terms. Additional information about protein complexes, such as disease associations and subunit composition, is now distributed among three comment fields. The database is continuously updated and expanded, with new data from various sources, including experimental and computational studies. CORUM is a valuable resource for researchers studying protein complexes in mammals and their evolutionary conservation.CORUM is a manually curated database of experimentally characterized protein complexes from mammalian organisms, mainly human (64%), mouse (16%), and rat (12%). The CORUM 2.0 release includes 2837 protein complexes, representing approximately 16% of human protein-coding genes. Each complex is described by name, subunit composition, function, and literature references. Recent developments include mapping to Gene Ontology terms and Entrez Gene identifiers, as well as a 'Phylogenetic Conservation' tool for predicting orthologous subunits in different organisms. CORUM is freely accessible at http://mips.helmholtz-muenchen.de/genre/proj/corum/index.html. The database provides a comprehensive dataset of mammalian protein complexes, with a focus on their subunit composition, function, and disease associations. It includes a 'Core Set' of 2084 distinct complexes to avoid redundancy. CORUM is used for various bioinformatics analyses, including tissue-specific expression, functional interpretation of high-throughput data, and predicting protein interactions. It also contributes to web-based applications like the DICS database and COFECO tool. The CORUM dataset has been used to analyze the composition and reuse of protein complex subunits, showing that most proteins are part of only one complex, while some are reused in multiple complexes. The 'Phylogenetic Conservation' tool allows prediction of orthologous subunits in different organisms, based on sequence similarity data from the SIMAP database. This tool helps in understanding the evolutionary conservation of protein complexes. CORUM is the only resource that includes functional annotation of protein complexes using the FunCat scheme. It also provides mappings from FunCat to Gene Ontology (GO) terms. Additional information about protein complexes, such as disease associations and subunit composition, is now distributed among three comment fields. The database is continuously updated and expanded, with new data from various sources, including experimental and computational studies. CORUM is a valuable resource for researchers studying protein complexes in mammals and their evolutionary conservation.