Cell Counting Kit-8 is an assay which contains the tetrazolium salt WST-8 (2-(2-methoxy-4-nitrophenyl)-3-(4-nitrophenyl)-5-(2,4-disulfophenyl)-2H-tetrazolium, monosodium salt). This tetrazolium salt produces a water-soluble formazan dye upon bioreduction in the presence of an electron carrier, 1-Methoxy PMS. Cell Counting Kit-8 is a sensitive nonradioactive colorimetric assay for determining the number of viable cells in cell growth and cytotoxicity assays. WST-8 is bioreduced by cellular dehydrogenases to an orange formazan product that is soluble in tissue culture medium. The amount of formazan produced is directly proportional to the number of living cells. The detection sensitivity of cell proliferation assays using WST-8 is higher than assays using the other tetrazolium salts such as