The ligand interaction scan (LIScan) method is a general procedure for engineering small molecule ligand–regulated forms of a protein that is complementary to other 'reverse' genetic and chemical-genetic methods for drug-target validation. It involves insertional mutagenesis by a chemical-genetic 'switch', comprising a genetically encoded peptide module that binds with high affinity to a small-molecule ligand. We demonstrated the method with TEM-1 β-lactamase, using a tetracysteine hexapeptide insert and a biarsenical fluorescein ligand (FlAsH).