PubChem contains the chemical structures of small organic molecules and information on their biological activities. PubChem includes substance information, compound structures, and bioactivity data in three primary databases, PCSubstance, PCCompound, and PCBioAssay, respectively. 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.