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Three linked databases within the National Center for Biotechnology Information Entrez information retrieval system. You can search these databases by names, synonyms, keywords: PubChem Substance; PubChem Compound; PubChem BioAssay.
ChemSpider is a free access service providing a structure centric community for chemists. Providing access to millions of chemical structures and integration to a multitude of other online services ChemSpider is the richest single source of structure-based chemistry information.