The Conserved Domain Database (CDD) is a protein classification component of NCBI's Entrez query and retrieval system, linked to other databases such as Proteins, Taxonomy, and PubMed. CDD provides a fast, interactive tool called CD-Search to identify conserved domains in new protein sequences. The database is updated and curated to reduce redundancy and improve functionality. CDD contains models from Pfam, SMART, COG, and NCBI-curated models, with pre-calculated CD-Search results available for proteins in Entrez. Users can explore domain hierarchies, sequence trees, and taxonomic diversity, and access alignment data and analysis algorithms. Future developments include distributing software for visualizing CD family hierarchies and sequence trees.The Conserved Domain Database (CDD) is a protein classification component of NCBI's Entrez query and retrieval system, linked to other databases such as Proteins, Taxonomy, and PubMed. CDD provides a fast, interactive tool called CD-Search to identify conserved domains in new protein sequences. The database is updated and curated to reduce redundancy and improve functionality. CDD contains models from Pfam, SMART, COG, and NCBI-curated models, with pre-calculated CD-Search results available for proteins in Entrez. Users can explore domain hierarchies, sequence trees, and taxonomic diversity, and access alignment data and analysis algorithms. Future developments include distributing software for visualizing CD family hierarchies and sequence trees.