Fig. 2. Relationship between chemical compounds and their targets for chemical probes versus chemogenomic compound sets. Chemical probes (CP)s are small molecules that are potent, selective and cell-active. Ideally, a chemical probe negative control is identified and used in parallel in phenotypic assays. A chemical probe ideally binds to a single target but can also be useful if it targets just a small number (∼2–3) of very closely sequence-related proteins. A chemogenomics library (CGL) comprises chemogenomic (CG) compounds of different chemotypes for each target; the chemotypes have complementary selectivity profiles, and preferably distinct modes of action (such as orthosteric or allosteric modulation). In the schematic, a CGL was screened against targets A–J yielding phenotypic data for 8 targets. These data are interpreted in the context of the magnitude of the phenotypic response, the mode of action (colour of arrow) and the CG chemical structures and accompanying characterisation.