The vast majority of the data in BindingDB are experimentally measured affinities of well-defined proteins, termed targets, with small organic molecules, termed compounds or ligands. However, BindingDB also contains small collections of protein-protein and host-guest binding affinities, as well as isothermal calorimetric and kinetic binding data. The data are organized into entries, where each entry is typically associated with a single scientific article or a single pending or granted patent. Thus, the data in one entry generally come from a single institution and are measured in the same manner. Each entry is assigned its own DOI with a defined landing page; for example, the BindingDB DOI 10.7270/Q2H993SB resolves to .) Binding affinities are typically expressed in terms of concentrations, most frequently as an IC50, an EC50, a dissociation constant (Kd), or an inhibition constant (Ki) expressed in nanomoles/L (nM). In all cases, a lower value implies a higher affinity. A given binding measurement pertains to a given protein target, a given compound, a description of the assay used, and the authors, institution (e.g., a university or company), publication information (e.g., a PubMed ID or a US patent number), and any other information associated with the entry (Figure ). BindingDB curation procedures also add annotations of entries with additional information, such as the identities of co-crystal structures available in the Protein Data Bank , the UniProt ID of the target , and compound links to chemical database and commercial chemical catalogs. BindingDB does not curate experimental three-dimensional structure data, instead providing users with links to the RCSB PDB for available structures of protein-ligand complexes, based on the chemical identity of the ligand and the sequence identity of the protein.