FAD-linked oxidoreductase; part of the gene cluster that mediates the biosynthesis of the cytotoxic leucine-containing cytochalasans, including aspochalasin C, aspochalasin E, TMC-169, flavichalasine F, aspergillin PZ, aspochalasin M and flavichalasine G (PubMed:32913332). The first step in the pathway is catalyzed by the hybrid PKS-NRPS ffsA that utilizes 8 units of malonyl-CoA to iteratively assemble the octaketide chain before addition of L-leucine by the C-terminal NRPS modules (PubMed:32913332). Because ffsA lacks a designated enoylreductase (ER) domain, the required activity is provided the enoyl reductase fssC (Probable). The methyltransferase (MT) domain of ffsA catalyzes the alpha-methylation at C10 and C14 using S-adenosyl-L-methionine as the methyl-donating cosubstrate (Probable). Reduction by the hydrolyase ffsE, followed by dehydration and intra-molecular Diels-Alder cyclization by the Diels-Alderase ffsF then yield the required isoindolone-fused macrocycle (By similarity). A number of oxidative steps catalyzed by the tailoring cytochrome P450 monooxygenase ffsD, the FAD-linked oxidoreductase ffsJ and the short-chain dehydrogenase/reductase ffsI, are further required to afford the final products (Probable). Mycotoxin biosynthesis. Belongs to the oxygen-dependent FAD-linked oxidoreductase family.