IPTG (isopropyl-beta-D-thiogalactopyranoside) is an analog of lactose. It inactivates the lac repressor and induces the synthesis of beta-galactosidase, an enzyme that promotes lactose utilization. This product is a 50 mg/ml solution by dissolving 250mg of IPTG in 5 ml of distilled H2O. IPTG is used to induce the expression of cloned genes under the control of the lac operon. It is used in conjunction with X-Gal to determine the lac phenotype in blue/white colony screening.