Fumonisins are mycotoxins produced by the corn pathogen Fusarium verticillioides. The toxins inhibit ceramide synthases, key enzymes in sphingolipid biosynthesis. We have been studying the molecular mechanism for fumonisin biosynthesis using genetic and biochemical approaches. The highly reduced, linear, dimethylated 18-carbon backbone of fumonisins is synthesized by an iterative modular polyketide synthase, which is encoded by FUM1, one of the 15 clustered genes that are co-expressed in this fungus. The covalently attached polyketide intermediate is released from the synthase by the decarboxylative addition of alanine, which is likely catalyzed by the L-serine-palmitoyltransferase-like enzyme encoded by FUM8. The released 3-keto intermediate is then reduced to a 3-hydroxy intermediate by the action of an NADPH-dependent ketoreductase coded by FUM13. Following the vicinal diol formation on C-14 and C-15, which is most likely catalyzed by the fused P450-monooxygenase-reductase coded by FUM6, two tricarballylic esters are introduced on C-14 and C-15. Interestingly, the ester bonds are made by an unusual nonribosomal peptide synthetase complex, consisting of at least three enzymes coded by FUM10, FUM7, and FUM14. Finally, the two hydroxyls on C-10 and C-5 are introduced by a second P450 monooxygenase coded by FUM12 and a 2-ketoglutarate-dependent dioxygenase coded by FUM3, respectively. We have also started to investigate the resistance mechanism. Within the FUM cluster, FUM17 and FUM18 are highly similar to ceramide synthase genes; FUM19 is highly similar to ABC transporter genes. However, the deletion of these genes did not affect the de novo fumonisin and sphingolipid biosynthesis.