IPTG (Isopropyl β-D-1-thiogalactopyranoside) is a useful molecular biology reagent. IPTG is a molecular mimic of allolactose that initiates transcription of the lac operon.  The key feature of IPTG is that, unlike allolactose, is not hydrolyzed by ß-galactosidase, therefore its concentration remains constant during an experiment. IPTG is therefore able to induce protein expression where the gene is under the control of the lac operator, when used as an effective inducer in the concentration range of 100μM to 1.5mM.