MTS reagent is one of the most common water-soluble tetrazolium reagents that are widely used to quantify cell viability by absorption spectrophotometry. A variety of tetrazolium inner salts are used for determining cell viability in proliferation assays by monitoring their color changes. Metabolically active cells reduce colorless tetrazolium inner salts into the colored formazan products that might be either soluble or insoluble in cell culture media depending on the structures of tetrazolium reagents. The conversion of tetrazolium inner salts to formazans is thought to be carried out by NAD(P)H-dependent dehydrogenase enzymes.