PubChem is a freely available set of three linked databases provided by the National Center for Biotechnology Information out of the National Library of Medicine. It includes PubChem Compound, PubChem Substance, and PubChem BioAssay. The chemical structure records within PubChem include links to other NCBI databases, including PubMed and Structure (protein 3D structure resource). The search box below allows you to search the three PubChem databases. Be sure to choose the one you would like to use from the drop-down menu before you click "Go."