The Conserved Domain Database (CDD) is a compilation of multiple sequence alignments representing protein domains conserved in molecular evolution. It has been populated with alignment data from the public collections Pfam ( 1) and Smart ( 2), as well as with contributions from colleagues at NCBI. The current version of CDD (v1.53) contains 3551 such models. CDD alignments are linked to protein sequence and structure data in Entrez (3 ). The molecular structure viewer Cn3D (4 ) serves as a tool to interactively visualize alignments and three dimensional structure, and to annotate three dimensional residue coordinates with evolutionarily conserved features.