A small molecule that triggers gene transcription by binding to a regulator protein (cf. enzyme induction). It acts by combining with the corresponding repressor protein to bring about an allosteric change so that the repressor is made incapable of combining with the operator of the system.
Source:
PAC, 1992, 64, 143. 'Glossary for chemists of terms used in biotechnology (IUPAC Recommendations 1992)' on page 157 (https://doi.org/10.1351/pac199264010143)