Cyclooxygenase catalyzes the first step in the biosynthesis of prostaglandins, thromboxanes, and prostacyclins: the conversion of arachidonic acid to prostaglandin H2. Recent discoveries of the induction of cyclooxygenase biosynthesis by a variety of stimuli such as phorbol esters, lipopolysaccharides, and cytokines led to the hypothesis that the inducible form of cyclooxygenase, COX-2, is responsible for the biosynthesis of prostaglandins under acute inflammatory conditions. COX-2 is a 72kDa protein which has been cloned from a variety of species including human, mouse, rat, and sheep.