Nitric oxide (NO) exerts a number of diverse activities in phylogenetically different species as well as opposing effects in related biological systems. This expression of a wide variety of effects is achieved through its interactions with targets via a rich redox and additive chemistry. Both covalent modifications of proteins as well as oxidation events that do not involve attachment of the NO group have been adopted as signaling mechanisms. One way to understand this versatility is to appreciate that NO has incorporated means of communication utilized by both protein kinases, which control function by covalent modification, ie, phosphorylation, and reactive oxygen species (ROS), which signal through both redox events and coordinative interactions with metals. In many cases, NO-responsive targets serve both sensory and regulatory roles in transducing a signal; that is, the target functions both in molecular recognition of NO and in the switching mechanism that transduces the chemical signal into a functional response (O’Halloran, 1993).