XTT Cell Viability Assay Kit provides a simple method for determination of live cell number using standard microplate absorbance readers. Determination of live cell number is often used to assess rate of cell proliferation and to screen cytotoxic agents. XTT is a tetrazolium derivative. Similar to MTT, XTT measures cell viability based on the activity of mitochondria enzymes in live cells that reduce XTT and are inactivated shortly after cell death. Unlike the water-insoluble formazan produced from MTT, XTT is readily reduced to a highly water-soluble orange colored product (1,2), thus omitting the solubilization step required for the MTT assay. The amount of water-soluble product generated from XTT is proportional to the number of living cells in the sample and can be quantified by measuring absorbance at wavelength of 475 nm.