Cell Counting Kit-8 (CCK-8) utilizes highly water-soluble tetrazolium salt WST-8:[2-(2-methoxy-4-nitrophenyl)-3-(4-nitrophenyl)-5-(2,4-disulfophenyl)-2H-tetrazolium, monosodium salt] changed to a water-soluble orange formazan dye by reduction. CCK-8 is low toxicity and allows sensitive colorimetric assays for the determination of the number of viable cells in cell proliferation and cytotoxicity assays. The amount of the formazan dye generated by dehydrogenases in cells is directly proportional to the number of living cells.