IPTG, Dioxane-Free (isopropyl-β-thiogalactoside) is an analogue of allolactose that binds specifically to the repressor protein of the lac operon and induces expression of β-galactosidase in Escherichia coli. The compound is widely used with X-Gal plates to screen for blue and white colonies. The lac operon system and IPTG have been used in multiple cloning experiments and cellular systems to study gene activity.