Fumonisin B1 (Macrofusine) is a mycotoxin produced by Fusarium moniliforme Sheldon and a potent inhibitor of sphingosine N-acyltransferase (ceramide synthase), an enzyme vital for the de novo synthesis of sphingolipids. It exhibits an IC50 of 0.1 µM for N-acyltransferase (ceramide synthase) in rat liver microsomes and also reduces the conversion of [3H]sphingosine to [3H]ceramide in intact hepatocytes.