Comparative statistics of the four are shown below in Table . As for the commercial sources, comparing content statistics between databases is not straightforward because the numbers in Figure are generated in slightly different ways. Not all the nuances can be addressed here but some salient ones can be pointed out. Moving across the columns there an element of circularity in compounds. The first reason is that ChEMBL subsumes 0.53 million compounds from confirmed PubChem BioAssays and 1.3 million curated from papers. The second is that BindingDB and ChEMBL have a mirroring collaboration in both directions where BindingDB subsumes the protein target assay results from ChEMBL and the latter subsumes BindingDB patent extraction data (e.g. the 137,000 compounds in release 25). This is separated from their total data counts in rows three and four. It also means that the overlap of compound structures, target and document identifiers between the two sources have extensive circularity (but some are independently curated). The PubChem figures for bioactivities seem large because these are factored by substances not compounds, whereas ChEMBL (by far the dominant contributor to PubChem BioAssay) collapses their assay counts to compounds. For GtoPdb the lower count reflects the curation of only compounds with curated binding constants from papers.