The National Center for Biotechnology Information's Protein Clusters Database. - William Klimke; Richa Agarwala; Azat Badretdin; Slava Chetvernin; Stacy Ciufo; Boris Fedorov; Boris Kiryutin; Kathleen O'Neill; Wolfgang Resch; Sergei Resenchuk; Susan Schafer; Igor Tolstoy; et al. (Nucleic Acids Res., 2009)