Abstract. The primary mission of UniProt is to support biological research by maintaining a stable, comprehensive, fully classified, richly and accurately annotated protein sequence knowledgebase, with extensive cross-references to external resources, that is freely available to the scientific community. To enable users of the knowledgebase to accurately assess the reliability of the information contained in this resource, the evidence for and provenance of the information must be recorded. This paper discusses the user requirements for this kind of metadata and the manner in which UniProtKB records it. 1 The UniProt Knowledgebase The UniProt Knowledgebase (UniProtKB) [1] consists of two parts: UniProtKB/ Swiss-Prot, containing manually annotated records describing proteins with information extracted from the scientific literature and curator-evaluated computational analysis, and UniProtKB/TrEMBL, with automatically annotated records. The UniProtKB is available in several data formats,...