<p>Flavin-containing monooxygenases (FMOs) constitute a family of xenobiotic-metabolising enzymes [<cite idref="PUB00000158"/>]. Using an NADPH cofactor and FAD prosthetic group, these microsomal proteins catalyse the oxygenation of nucleophilic nitrogen, sulphur, phosphorous 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 [<cite idref="PUB00002642"/>, <cite idref="PUB00002611"/>,<cite idref="PUB00004772"/>, <cite idref="PUB00000516"/>, <cite idref="PUB00002834"/>]. 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 [<cite idref="PUB00000158"/>]. Following the determination of the complete nucleotide sequence of <taxon tax_id="4932">Saccharomyces cerevisiae</taxon> (Baker's yeast) [<cite idref="PUB00005185"/>], a novel gene was found to encode a protein with similarity to mammalian monooygenases.</p>