The Human Metabolome Database (HMDB) (http://www.hmdb.ca)20-22 is comprehensive information on human metabolites and human metabolism data. This database contains curated information derived from scientific literature, as well as experimentally determined metabolite concentrations in human tissue or biofluid (e.g., urine, blood, cerebrospinal fluid and so on). Reference Mass spectra (MS) and nuclear magnetic resonance (NMR) spectra for metabolites are also provided when available. In addition to data for “detected” metabolites (those with measured concentrations or experimental confirmation of their existence), the HMDB also provides information on “expected” metabolites (those for which biochemical pathways are known or human intake/exposure is frequent but the compound has yet to be detected in the body).