Formazan dyes are artificial chromogenic substrates for dehydrogenases and reductases. They include MTT (3-(4, 5-dimethyl-2-thiazolyl)-2, 5-dihphenyl-2H-tetrazolium bromide) (used in the MTT assay), and the more water soluble XTT (23-bis-(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide). When reduced in a cell, either enzymatically or through direct reaction with NADH or NADPH MTT turns bright blue and may form an insoluble precipitate.