The reduction of tetrazolim salts to colored formazan compounds by succinate-tetrazolim reductase, which exists in viable cells, provides a sensitive and accurate method to measure cell viability and proliferation. The most commonly used tetrazolim salt, MTT [3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide], however, suffers from the disadvantage that the formazan dye produced from MTT is extremely water insoluble, so an additional extraction step is needed for spectrophotometric quantification. Instead of MTT, the ScienCell WST-1 Cell Viability & Proliferation Assay utilizes a tetrazolim salt WST-1[2-(4-Iodophenyl)-3-(4-nitrophenyl)-5-(2,4-disulfophenyl)-2H-tetrazolium]. WST-1 produces a highly water soluble formazan upon metabolically active cells, allowing a direct and user-friendly colorimetric measurement of cell viability and proliferation.