The Universal Protein Resource (UniProt) is a comprehensive resource for protein sequence and functional annotation data. UniProt comprises four major components, each optimised for different uses. The UniProt Knowledgebase (UniProtKB) is an expertly curated database, and a central access point for integrated protein information with cross-references to multiple sources. The UniProt Archive (UniParc) is a comprehensive sequence repository, reflecting the history of all protein sequences. UniProt Reference Clusters (UniRef) merge closely related sequences based on sequence identity to speed up searches.