The talk will describe studies on the underlying mechanisms by which humans and other animals respond to changes in oxygen availability, e.g. when we climb a mountain or in cells that are rapidly dividing as in tumours. Remarkably, the 'oxygen-sensors' are related to the enzymes that are responsible for the biosynthesis of penicillin. The possibility of manipulating the sensing enzyme for therapeutic benefit will be discussed as well as the chemical basis of their action.