COX-3 is an enzyme that is encoded by the PTGS1 (COX1) gene, but is not functional in humans. COX-3 is the third and most recently discovered cyclooxygenase (COX3050) isozyme, the others COX-1 and COX-2. The COX-3 isozyme is encoded by the same gene as COX-1, with the difference that COX-3 retains an intron that is not retained in COX-1. The other two cyclooxygenase isozymes are known to convert dihomo-γ-linolenic acid and arachidonic acid into prostaglandins, and are the targets of nonsteroidal anti-inflammatory drugs (NSAIDs). Transcription COX-3 is transcribed from the PTGS1 (COX1) gene, but the resulting mRNA is spliced differently. In dogs the resulting protein resembles the other two COX enzymes, but in mice and humans it does not, owing to a frame-shift mechanism. This mechanism is due to the fact that the spliced intron has 93 bases in dogs, resulting in the loss of 93:3 = 31 amino acids in the COX-3 sequence, which apparently does not impair its functionality. In humans, the intron is 94 bases long, leading to a protein with a completely different amino acid sequence from those of COX-1 or COX-2. The expressed protein does not show COX activity, and it is unlikely to play a role in prostaglandin-mediated physiological responses. Discovery The original COX-1/COX-2 model did not fully explain the immune responses of fever and inflammation. Even though COX-2 inhibitors are as active as traditional NSAIDs in inflammatory models, there were still some unexplained issues. For example, the widespread use of the newer generation of COX-2-selective compounds demonstrated that COX-2 also has other physiological roles, e.g. in the maintenance of fluid balance by the kidneys. In addition, the COX-1/COX-2 model did not explain the properties of paracetamol (acetaminophen): although its antipyretic (fever reducing) and analgesic (pain relieving) effects might be explained by inhibition of COX-2, it is not anti-inflammatory. Daniel L. Simmons' group suggested this was because of the presence of a variant of COX-1, which they named COX-3, that would be especially sensitive to paracetamol and related compounds. If this enzyme were particularly expressed in the brain, it could explain both the characteristics of paracetamol, which has been reputed for some time of being a centrally-acting antipyretic. COX-3 was actually discovered in 2002, and been found to be selectively inhibited by paracetamol, phenacetin, antipyrine, dipyrone, and some NSAIDs in rodent studies. Acetaminophen is thought of as a mild analgesic and antipyretic suitable, at