The biosynthetic stepsbetween 5 and the penultimate product demethylmycophenolicacid (DMMPA; 6) have been speculated (14, 23) but remainuncharacterized, while the final step is proposed to be the O-methylation of the C5 hydroxy group of 6 by the OmethyltransferaseMpaG′ (15) to yield the final product 1.