Short-chain dehydrogenase/reductase, part of the gene cluster that mediates the biosynthesis of melleolides, a range of antifungal and phytotoxic polyketide derivatives composed of an orsellinic acid (OA) moiety esterified to various sesquiterpene alcohols (Probable). The first step in melleolides biosynthesis is performed by the delta(6)-protoilludene synthase PRO1 which catalyzes the cyclization of farnesyl diphosphate to protoilludene. The orsellinic acid synthase armB produces OA by condensing acetyl-CoA with 3 malonyl-CoA units in a three-round chain elongation reaction folowed by a C2-C7 ring closure. ArmB further catalyzes the trans-esterification of OA to the various sesquiterpene alcohols resulting from the hydroxylation of protoilludene. The melleolides cluster also includes 5 cytochrome P450 monooxygenases, 4 NAD(+)-dependent oxidoreductases, one flavin-dependent oxidoreductase, and one O-methyltransferase. The cytochrome P450 monooxygenases may be involved in protoilludene hydroxylation to elaborate melleolides with multiple alcohol groups, such as melleolide D, which carries alcohol functionalities at C-4, C-5, C-10, and C-13. The role of the NAD(+)-dependent enzymes remains unknown. Numerous melleolides, including arnamial, show 5'-O-methylation of the aromatic moiety which may be catalyzed by the methyltransferase encoded in the cluster. The flavin-dependent oxidoreductase might represent the dehydrogenase yielding the aldehyde in position 1 of arnamial and other melleolides. Finally, several halogenase localized outside of the cluster, are able to catalyze the transfer of a single chlorine atom to the melleolide backbone, resulting in a 6'-chloromelleolide product 