The prostanoid family includes PGD2, PGE2, PGF2alpha, PGI2, thromboxane A2 and prostaglandins. The prostaglandins (PGs) are implicated in various physiological and pathophysiological events, including male fertility, menstruation, ovulation, pregnancy, implantation and inflamatory and neoplastic diseases. The biosynthesis of PGs and some other prostanoids is catalyzed in a rate limiting step by PG-H synthase (also known as cyclooxygenase (COX), PG-endoperoxidase synthase (PTGS)) which converts arachiodonic acid to prostaglandin/prostanoid precursor PGH2. Two cyclooxygenase isozymes, COX1 (human, 576aa, 69-72kD; chromosome 9) and COX2 (human, 604aa, 74kD; chromosome 1) have been identified. COX1 is a constitutively expressed isoform. it produces physiologically relevant prostanoids such as those in stomach and platelets. COX2 isoform is inducible. it is rapidly upregulated at inflamation sites. It forms proinflamatory prostanoids. The overexpression of COX2 also leads to tumerogenesis. Recently, a third isoform COX3 (canine 633aa; ~65kD in human aorta) has been reported. Two smaller COX1-derived proteins (partial COX1) PCOX1a (canine 414aa, ~53kD in human aorta) and PCOX1b have also been characterized. The COX3, but not PCOX1a, possesses glycosylation dependent cyclooxygenase activity. The nonsteroidal anti-inflammatory drugs (NSAIDs) reduce the formation of prostaglandins by inhibiting the activity of cyclooxygenases (COX1, COX2 and COX3). This ability was associated with inhibition of COX, which converts arachidonic acid to the prostaglandin precursor prostaglandin H2. COX1 may play an important role in regulating or promoting cell proliferation in some nromal and nepotistic cells. it catalyzes arachidonate + AH2 + 2 O2 = Prostaglandin H2 + A + H2O. This enzyme acts both as a dioxygenase and as a peroxidase. It acts as the first step in the formation of prostaglandins and thromboxanes. COX1 contains 1 EFG-like domain. It consists of 70kD subunits which are constitutively expressed. These subunits are involved in a range of physiological functions. Inhibition of gastric expressed COX1 resulted in most of the unwanted side effects. Non-steroidal inflammatory drugs (NSAIDs) treated the symptoms.