mono-tetrazolium salt made up of three aromatic rings, comprising two phenyl moieties and one thiazolyl ring, surrounding a positively charged quaternary tetrazole ring core with four nitrogen atoms. When MTT is reduced, the central tetrazole ring is broken, creating the violet-blue, water-insoluble chemical known as formazan. Due to its positive charge and lipophilic nature, the MTT reagent can pass through the mitochondrial inner membrane and cell membrane of living cells, and it is converted to formazan by metabolically active cells.