Isopropyl-beta-D-thiogalactopyranoside (IPTG) is a non-metabolizable allolactose analogue, widely used in molecular biology for overexpression of recombinant proteins from inducible systems under the control of lac promoter. IPTG binds to the LacI repressor and causes its release from the lac operator, allowing gene expression of lac operon to take place. IPTG is present in vectors of pGEX, pGEM-T, pET, pRSET, pMAL class and others.