Flavin-containing monooxygenases (FMOs) constitute a family of xenobiotic-metabolising enzymes. Using an NADPH cofactor and FAD prosthetic group, these microsomal proteins catalyse the oxygenation of nucleophilic nitrogen, sulphur, phosphorus and selenium atoms in a range of structurally diverse compounds. FMOs have been implicated in the metabolism of a number of pharmaceuticals, pesticides and toxicants. In man, lack of hepatic FMO-catalysed trimethylamine metabolism results in trimethylaminuria (fish odour syndrome). Five mammalian forms of FMO are now known and have been designated FMO1-FMO5. This is a recent nomenclature based on comparison of amino acid sequences, and has been introduced in an attempt to eliminate confusion inherent in multiple, laboratory-specific designations and tissue-based classifications. Following the determination of the complete nucleotide sequence of Saccharomyces cerevisiae (Baker's yeast), a novel gene was found to encode a protein with similarity to mammalian monooygenases. In Aspergillus, flavin-containing monooxygenases ustF1 and ustF2 are components in the biosynthesis of the antimitotic tetrapeptide ustiloxin B, a secondary metabolite. The monooxygenases modify the side chain of the intermediate S-deoxyustiloxin H.