Cell Counting Kit-8 (CCK-8) allows convenient assays using WST-8 (2-(2-methoxy-4-nitrophenyl)-3-(4-nitrophenyl)-5-(2,4-disulfophenyl)-2H-tetrazolium, monosodium salt), which produces a water-soluble formazan dye upon bioreduction in the presence of an electron carrier, 1-Methoxy PMS. CCK-8 solution is added directly to the cells, no pre-mixing of components is required. WST-8 is reduced by cellular dehydrogenases to an orange formazan product that is soluble in a tissue culture medium. The CCK-8 assay measures cell viability based on formazan production, which is directly proportional to the number of living cells. Since the CCK-8 solution is very stable and has little cytotoxicity, a longer incubation, such as 24 to 48 hours, is possible. Cell Counting Kit-8 allows sensitive colorimetric assays for the determination of the number of viable cells in the proliferation and cytotoxicity assays. The detection sensitivity of CCK-8 is superior to that of other tetrazolium salts, including MTT, XTT, and MTS. Figure 1: Working mechanisms of Cell Counting Kit-8 (CCK-8).