Normally, cyclooxygenases (COX) carry out stereospecific oxygenation of arachidonic acid to generate prostaglandins. When treated with aspirin (acetylsalicylic acid, ASA), dimeric cyclooxygenase-2 (COX2, PTGS2 dimer) can be acetylated. ASA covalently modifies PTGS2 by acetylating a serine residue at position 530 within the cyclooxygenase active site (Lucido et al. 2016). Acetylated PTGS2 dimer (Ac-PTGS2 dimer) changes the oxygenation stereospecificity towards its substrates, perhaps by steric shielding effects (Tosco 2013), producing a shift in lipid mediator production. Ac-PTGS2 dimer is able to incorporate molecular oxygen into ω-3 fatty acid docosahexaenoic acid (DHA), present in inflammatory exudates, to form the 17(R) epimer 17(R)-hydroperoxy-docosahexaenoic acid (17(R)-Hp-DHA) (Serhan et al. 2002, Sun et al. 2007). The product can either be transformed into aspirin-triggered D-resolvins or aspirin-triggered protectin D1 (Serhan et al. 2015).