The most studied FMO is the human Fmo3, an enzyme that is constitutively active in the adult human liver where it participates in the oxidation of drugs and xenobiotics, and mutations in Fmo3 contribute to the disease trimethylaminuria (Cashman and Zhang, 2002; Dolphin et al., 1996; Krueger and Williams, 2005). The gene discussed is FMO3; the disease is trimethylaminuria.