UniProt (Universal Protein Resource) is a comprehensive catalog of information on proteins. It is a central repository of protein sequence and function created by joining the information contained in Swiss-Prot, TrEMBL, and PIR. UniProt is comprised of three components, each optimised for different uses. The UniProt Knowledgebase (UniProtKB) is the central access point for extensive curated protein information, including function, classification, and cross-reference. The UniProt Non-redundant Reference (UniRef) databases combine closely related sequences into a single record to speed searches. The UniProt Archive (UniParc) is a comprehensive repository, reflecting the history of all protein sequences. The UniProt Knowledgebase contains protein data from all species where it is available. This data includes protein sequences determined by direct experiment and derived from the sequencing of individual DNA clones or RNA molecules. It does not, however, necessarily include predictions of