Having assigned target IDs to both probe sets we compared these with informative cross-references in UniProt. We selected 4,213 protein IDs (as human Swiss-Prot entries) based on the union (OR operator) of the four high-quality curated chemistry-to-target databases (already mentioned) of ChEMBL, BindingDB, GtoPdb and DrugCentral. These IDs represent liganded targets for 21% of the UniProt proteome of 20,395 (although this drops to 19,205 for HUGO Gene Nomenclature Committee annotation). The comparative protein sets we also selected were from the four Target Development Levels (TDLs) of the Pharos resource for Illuminating the Druggable Genome (IDG). This facilitates exploration of both the characterised and the understudied (or "dark") regions of the human proteome with a view to expanding functional insights and finding new drug targets. We initially selected the combination of the Tchem (1,593 proteins) known to bind small molecules (other than approved drugs) with target-class specific potency thresholds plus the Tclin 54 (659 proteins) as targets of approved drugs. The union of these two is 2,221 proteins. The intersections of these four lists are shown in Figure .