The Human Metabolome Database is a freely available database containing detailed information about small-molecule metabolites found in the human body, including many with measured concentration values. It is designed to contain or link three kinds of data: chemical, clinical, and molecular biology/biochemistry data. HMDB contains over 200,000 metabolite entries. It also has specialized sub-databases that are listed below. Each metabolite entry contains more than 110 data fields and many of them are hyperlinked to other databases (e.g., KEGG, PubChem, Uniprot).