o-Nitrophenyl-β-D-galactoside (ONPG) is a colorimetric substrate for b-galactosidase. This product is hydrolyzed to o-nitrophenol by the enzyme. o-Nitrophenol is yellow in alkaline solution and can be quantitated at 410-420 nm 2 . o-Nitrophenyl-β-D-galactoside is a cell culture…