The XTT assay is an optimized colorimetric assay for assessing cell viability and proliferation in mammalian cells based on their cellular redox potential. Dehydrogenase enzymes in metabolically active cells reduce the yellow tetrazolium salt XTT to the brightly orange formazan dye which can be measured at 450 nm. Since this reduction can only occur in metabolically active cells, it is considered as an indicator of cell viability. The amount of produced formazan correlates with the number of viable cells in the sample, and an electron coupling reagent is used to improve the efficiency of XTT reduction in cells.