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Genetics Home Reference: your guide to understanding genetic conditions     A service of the U.S. National Library of Medicine®


Reviewed April 2007

What is the official name of the FMO3 gene?

The official name of this gene is “flavin containing monooxygenase 3.”

FMO3 is the gene's official symbol. The FMO3 gene is also known by other names, listed below.

What is the normal function of the FMO3 gene?

The FMO3 gene provides instructions for making an enzyme that is part of a larger enzyme family called flavin-containing monooxygenases (FMOs). These enzymes break down compounds that contain nitrogen, sulfur, or phosphorus. The FMO3 enzyme, which is made chiefly in the liver, is responsible for breaking down nitrogen-containing compounds derived from the diet. One of these compounds is trimethylamine, which is the molecule that gives fish their fishy smell. Trimethylamine is produced as bacteria in the intestine help digest certain proteins obtained from eggs, liver, legumes (such as soybeans and peas), certain kinds of fish, and other foods. The FMO3 enzyme normally converts fishy-smelling trimethylamine into another compound, trimethylamine-N-oxide, which has no odor. Trimethylamine-N-oxide is then excreted from the body in urine.

Researchers believe that the FMO3 enzyme also plays a role in processing some types of drugs. For example, this enzyme is likely needed to break down the anticancer drug tamoxifen, the pain medication codeine, the antifungal drug ketoconazole, and certain medications used to treat depression (antidepressants). The FMO3 enzyme may also be involved in processing nicotine, an addictive chemical found in tobacco. Normal variations (polymorphisms) in the FMO3 gene may affect the enzyme's ability to break down these substances. Researchers are working to determine whether FMO3 polymorphisms can help explain why people respond differently to certain drugs.

How are changes in the FMO3 gene related to health conditions?

trimethylaminuria - caused by mutations in the FMO3 gene

More than 25 mutations in the FMO3 gene have been identified in people with trimethylaminuria. Most of these mutations lead to the production of a small, nonfunctional version of the FMO3 enzyme. Other mutations change single building blocks (amino acids) used to build the enzyme, which alters its shape and disrupts its function. Without enough functional FMO3 enzyme, the body is unable to convert trimethylamine into trimethylamine-N-oxide effectively. As a result, trimethylamine builds up in the body and is released in an affected person's sweat, urine, and breath. The excretion of this compound is responsible for the strong body odor characteristic of trimethylaminuria. Studies suggest that diet and stress also play a role in determining the intensity of the fish-like odor.

Where is the FMO3 gene located?

Cytogenetic Location: 1q24.3

Molecular Location on chromosome 1: base pairs 171,090,877 to 171,117,821

(Homo sapiens Annotation Release 107, GRCh38.p2) (NCBI (

The FMO3 gene is located on the long (q) arm of chromosome 1 at position 24.3.

The FMO3 gene is located on the long (q) arm of chromosome 1 at position 24.3.

More precisely, the FMO3 gene is located from base pair 171,090,877 to base pair 171,117,821 on chromosome 1.

See How do geneticists indicate the location of a gene? ( in the Handbook.

Where can I find additional information about FMO3?

You and your healthcare professional may find the following resources about FMO3 helpful.

You may also be interested in these resources, which are designed for genetics professionals and researchers.

What other names do people use for the FMO3 gene or gene products?

  • Dimethylaniline monooxygenase [N-oxide-forming] 3
  • Dimethylaniline oxidase 3

See How are genetic conditions and genes named? ( in the Handbook.

What glossary definitions help with understanding FMO3?

acids ; antidepressants ; bacteria ; codeine ; compound ; depression ; enzyme ; excretion ; gene ; intestine ; ketoconazole ; metabolism ; molecule ; nicotine ; oxidase ; pharmacogenetics ; pharmacogenomics ; phosphorus ; stress ; trimethylamine

You may find definitions for these and many other terms in the Genetics Home Reference Glossary.


  • Bain MA, Fornasini G, Evans AM. Trimethylamine: metabolic, pharmacokinetic and safety aspects. Curr Drug Metab. 2005 Jun;6(3):227-40. Review. (
  • Dolphin CT, Janmohamed A, Smith RL, Shephard EA, Phillips IR. Missense mutation in flavin-containing mono-oxygenase 3 gene, FMO3, underlies fish-odour syndrome. Nat Genet. 1997 Dec;17(4):491-4. (
  • Hernandez D, Addou S, Lee D, Orengo C, Shephard EA, Phillips IR. Trimethylaminuria and a human FMO3 mutation database. Hum Mutat. 2003 Sep;22(3):209-13. Review. (
  • Koukouritaki SB, Poch MT, Henderson MC, Siddens LK, Krueger SK, VanDyke JE, Williams DE, Pajewski NM, Wang T, Hines RN. Identification and functional analysis of common human flavin-containing monooxygenase 3 genetic variants. J Pharmacol Exp Ther. 2007 Jan;320(1):266-73. Epub 2006 Oct 18. (
  • Krueger SK, Vandyke JE, Williams DE, Hines RN. The role of flavin-containing monooxygenase (FMO) in the metabolism of tamoxifen and other tertiary amines. Drug Metab Rev. 2006;38(1-2):139-47. Review. (
  • Mitchell SC. Trimethylaminuria: susceptibility of heterozygotes. Lancet. 1999 Dec 18-25;354(9196):2164-5. (
  • NCBI Gene (
  • Zhang J, Tran Q, Lattard V, Cashman JR. Deleterious mutations in the flavin-containing monooxygenase 3 (FMO3) gene causing trimethylaminuria. Pharmacogenetics. 2003 Aug;13(8):495-500. (
  • Zhou J, Shephard EA. Mutation, polymorphism and perspectives for the future of human flavin-containing monooxygenase 3. Mutat Res. 2006 Jun;612(3):165-71. Epub 2006 Feb 14. Review. (
  • Zschocke J, Kohlmueller D, Quak E, Meissner T, Hoffmann GF, Mayatepek E. Mild trimethylaminuria caused by common variants in FMO3 gene. Lancet. 1999 Sep 4;354(9181):834-5. (


The resources on this site should not be used as a substitute for professional medical care or advice. Users seeking information about a personal genetic disease, syndrome, or condition should consult with a qualified healthcare professional. See How can I find a genetics professional in my area? ( in the Handbook.

Reviewed: April 2007
Published: February 8, 2016