Isopropyl β-D-1-thiogalactopyranoside (IPTG) is a molecular biology reagent.This compound is a molecular mimic of allolactose, a lactose metabolite that triggers transcription of the lac operon, and it is therefore used to induce protein expression where the gene is under the control of the lac operator.or induction, a sterile, filtered 1 M solution of IPTG is typically added by 1:1000 dilution into a logarithmically growing bacterial culture, however, different final concentration of IPTG may also be used.