A combination of genetic and pharmacological approaches allows the accuracy of single mutations to be merged with the spatial and temporal control afforded by the use of small molecules. An example is the 'bump and hole' methodology that involves making a mutant protein with a smaller amino acid, or 'hole', which can accommodate a 'bumped' ligand that binds to the mutant, but not the original wild type protein (Figure ). This approach was pioneered by Schreiber using FKBP12 and cyclophilin, and Shokat who applied this approach to generating selective kinaseligand pairs. More recently, Ciulli has used a bump and hole approach to distinguish between the structurally similar first and second BET bromodomains. An alternative approach is to introduce a mutant cysteine residue to help confer selectivity of a ligand for the mutant protein within a given family. Figure . The concepts of (A) bump and hole, (B) tethering, and (C) mutate and conjugate, which all combine protein engineering with small molecules to enable identification of selective ligands for a mutant protein. Created with BioRender.com.