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 diversecompounds. Five mammalian forms of FMO are now known and have been designatedFMO1-FMO5 [<cite idref="PUB00002642"/>, <cite idref="PUB00002611"/>, <cite idref="PUB00004772"/>, <cite idref="PUB00000516"/>, <cite idref="PUB00002834"/>].<p>FMO4 mRNA is present in low abundance in several foetal and adult tissuesand the corresponding gene thus appears to be expressed constitutively [<cite idref="PUB00001464"/>].Sequence analysis reveals that FMO4 is 56% identical to FMO3; each isencoded by a single gene [<cite idref="PUB00002842"/>]. The deduced amino acid sequence of human FM04 includes the putative FAD- (GxGxxG) and NADP<sup>+</sup> pyrophosphate-binding (GxGxxA)sites characteristic of mammalian FMOs, a 'FATGY' motif that has also beenobserved in a range of siderphore biosynthetic enzymes [<cite idref="PUB00005489"/>], and a C-terminalhydrophobic segment that is believed to anchor the monooxygenase to themicrosomal membrane [<cite idref="PUB00002549"/>].</p>