Peroxynitrite (ONOO-), a product of the reaction of nitric oxide with superoxide, is a potent and versatile oxidant that can attack a wide range of biological molecules. The peroxynitrite anion is relatively stable under basic conditions but is protonated at physiological pH; under physiological conditions, the half-life of the protonated species, peroxynitrous acid, is about 1 second. The peroxynitrite anion can react with DNA, proteins, and lipids at physiological conditions, leading to cellular damage and cytotoxity. Reaction with DNA can lead to oxidation and/or nitration to initiate a complex series of transformations involving base damage or strand breaks as well as reactions with the deoxyribose portion of the DNA. A major reaction with proteins is the formation of nitrotyrosine.