Besides causing the decay of fruit, another important hazard of P.expansum is producing patulin. Patulin, a mycotoxin of polyketide metabolite, is notable for its extremely widespread and frequent occurrence in fruits and derived production. It has been suggested as a likely carcinogen due to its ability to interfere withDNA and RNA synthesis. The pathway leading to the production of patulin is thought to involve at least 10 different enzymatic steps. The first step is the formation of 6-methylsalicylic acid(6-MSA) by the condensation of one acetyl-CoA and three malonyl-CoA units. This formation is carried out by 6-methylsalicylic acid synthase (6-MSAS), a single multifunctional enzyme with acetyland malonyl transferase, ketoacyl synthase, ketoreductase and dehydratase activities (Puel et al., 2010). The seventh step of patulin production is the conversion of isoepoxydon to phyllostine,accomplished via an NADP-dependent isoepoxydon dehydroge-nase (IDH) (Dombrink-Kurtzman, 2006). Both 6-MSAS and IDHplayed an important role in patulin biosynthesis. Puel et al. (2007)reported that the inability of Byssochlamys fulva to produce patulinwas related to absence of 6-MSAS and IDH genes. In this study, asignificant down-regulated expression of msas and IDH genes in P.expansum after NaHCO3 treatment may well explain the reason fordecrease of patulin production.