Tetrazoles are a class of synthetic organic heterocyclic compound, consisting of a 5-member ring of four nitrogen and one carbon atom (plus hydrogens). The simplest is tetrazole itself, CN₄H₂. They are unknown in nature. There are several pharmaceutical agents which are tetrazoles, but they are generally undesirable due to safety concerns for process-scale synthesis; tetrazoles are usually explosive. However, tetrazoles can act as a bioisostere for the carboxylate group, increasing their utility. Angiotensin II receptor blockers, in particular, often contain tetrazoles, such as losartan and candesartan. A well-known tetrazole is MTT, which is a dimethyl thiazolyl diphenyl tetrazolium salt. This tetrazole is used in the MTT assay to quantify the respiratory activity of live cells in cell culture, although it generally kills the cells in the process.