<unk>  (nitrogen<unk>  or nitrogen monoxide) is a colorless gas with the formula . It is one of the principal<unk> s of nitrogen.<unk>  is a free radical: it has an unpaired electron, which is sometimes denoted by a dot in its chemical formula (•N=O or •NO).<unk>  is also a heteronuclear diatomic molecule, a class of molecules whose study spawned early modern theories of chemical bonding. An important intermediate in industrial chemistry,<unk>  forms in combustion systems and can be generated by lightning in thunderstorms. In mammals, including humans,<unk>  is a signaling molecule in many physiological and pathological processes. It was proclaimed the "Molecule of the Year" in 1992. The 1998 Nobel Prize in Physiology or Medicine was awarded for discovering<unk> 's role as a cardiovascular signalling molecule.<unk>  should not be confused with nitrogen dioxide (NO2), a brown gas and major air pollutant, nor with nitrous<unk>  (N2O), an anesthetic.
Nitric oxide
Leukotriene
Histamine
Serotonin
Acetylcholine
Interleukin
Nitrosamine
Prostaglandin