The MTT assay is a colorimetric assay for assessing [cell metabolic activity](). It is based on the reduction of the yellow tetrazolium dye MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) to purple formazan crystals by metabolically active cells. The process is catalyzed by [mitochondrial enzymes](), particularly succinate dehydrogenase, in living cells. Therefore, the assay is commonly used to measure cell viability, proliferation, and cytotoxicity.