XTT is a cell-impermeable, negatively charged tetrazolium dye that produces a water-soluble formazan when reduced at the cell surface by cellular-derived NADH and an electron mediator.1,2It is frequently used in colorimetric assays to measure cell proliferation, cytotoxicity, and apoptosis.3 1.Berridge, M.V., Tan, A.S., and Herst, P.M.Tetrazolium dyes as tools in cell biology: New insights into their cellular reductionBiotechnol. Ann. Rev.11127-152(2005) 2.Berridge, M.V., and Tan, A.S.Trans-plasma membrane electron transport: A cellular assay for NADH-and NADPH-oxidase based on extracellular, superoxide-mediated reduction of the sulfonated tetazolium salt WST-1Protoplasma20574-82(1998) 3.Sutherland, M.W., and Learmonth, B.A.The tetrazolium dyes MTS and XTT provide new quantitative assays for superoxide and superoxide dismutaseFree Radical Research27(3)283-289(1997)