<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"/>].</p><p>Human FMO1 mRNA is more abundant in foetal than in adult liver, indicating that the enzyme is subject to developmental regulation in man [<cite idref="PUB00002642"/>]. The deduced amino sequence contains putative FAD- (GxGxxG) and NADP⁺-binding (GxGxxA) sites, a 'FATGY' motif that has also been observed in a range of siderphore biosynthetic enzymes [<cite idref="PUB00005489"/>], and a C-terminal hydrophobic segment that is believed to anchor the monooxygenase to the microsomal membrane [<cite idref="PUB00002549"/>]. The human sequence shares 88 and 86% identity, respectively, with pig and rabbit 'hepatic' forms of FMO, but is only 58% similar to the abbit 'pulmonary' FMO [<cite idref="PUB00002642"/>].</p> Flavin monooxygenase (FMO) 1