For each target, we queried ChEMBL version 25 for ligands that bind to that target and which had one or more measurement of K i . We used only K i data and excluded measurements such as IC 50 so as not to conflate different types of data. We trained our models using the corresponding pChEMBL value, which is the negative base-10 logarithm of the binding constant reported by ChEMBL, equivalent to the pK value. Duplicate pChEMBL values for a protein-ligand pair were removed; for ligands with different pChEMBL values for the same target, we used the arithmetic mean of the pChEMBL value. The DUD-E abbreviations for the names of the six targets for which K i data were available in ChEMBL, together with the PDB code of the structure provided by DUD-E and the number of ligands for each target, are shown in Supporting Information Table .