PubChem (NCBI) provides information on the biological activities of small molecules. It is a component of NIH's Molecular Libraries Roadmap Initiative. PubChem includes substance information, compound structures, and BioActivity data in three primary databases, Pcsubstance, Pccompound, and PCBioAssay, respectively. The Substance/Compound database, where possible, provides links to BioAssay description, literature, references, and assay data points. The BioAssay database also includes links back to the Substance/Compound database. PubChem is integrated with Entrez, NCBI's primary search engine, and also provides compound neighboring, sub/superstructure, similarity structure, BioActivity data, and other searching features. PubChem contains substance and BioAssay information from a multitude of depositors. Registration not required.