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


Reviewed July 2009

What is the official name of the UROS gene?

The official name of this gene is “uroporphyrinogen III synthase.”

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

What is the normal function of the UROS gene?

The UROS gene provides instructions for making an enzyme known as uroporphyrinogen III synthase. This enzyme is involved in the production of a molecule called heme. Heme is vital for all of the body's organs, although it is most abundant in the blood, bone marrow, and liver. Heme is an essential component of iron-containing proteins called hemoproteins, including hemoglobin (the protein that carries oxygen in the blood).

The production of heme is a multi-step process that requires eight different enzymes. Uroporphyrinogen III synthase is responsible for the fourth step in this process, in which hydroxymethylbilane (the product of the third step) is rearranged to form uroporphyrinogen III. In subsequent steps, four other enzymes produce and modify compounds that ultimately lead to heme.

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

porphyria - caused by mutations in the UROS gene

More than 35 mutations in the UROS gene have been found to cause a form of porphyria known as congenital erythropoietic porphyria. Most of these mutations change single protein building blocks (amino acids) in uroporphyrinogen III synthase. The most common UROS gene mutation, which is found in about one-third of all cases of congenital erythropoietic porphyria, replaces the amino acid cysteine with the amino acid arginine at position 73 (written as Cys73Arg or C73R). Several other mutations occur in a nearby region of DNA that regulates the activity of the UROS gene.

Mutations in or near the UROS gene alter the structure and function of uroporphyrinogen III synthase, which reduces the enzyme's activity. A shortage of functional uroporphyrinogen III synthase allows compounds called porphyrins to build up in developing red blood cells. These compounds are formed during the normal process of heme production, but reduced activity of uroporphyrinogen III synthase allows them to accumulate to toxic levels. The excess porphyrins can leak out of developing red blood cells and be transported through the bloodstream to the skin and other tissues. An accumulation of these substances in the skin causes oversensitivity to sunlight and the other characteristic features of congenital erythropoietic porphyria.

Where is the UROS gene located?

Cytogenetic Location: 10q25.2-q26.3

Molecular Location on chromosome 10: base pairs 125,784,980 to 125,823,268

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

The UROS gene is located on the long (q) arm of chromosome 10 between positions 25.2 and 26.3.

The UROS gene is located on the long (q) arm of chromosome 10 between positions 25.2 and 26.3.

More precisely, the UROS gene is located from base pair 125,784,980 to base pair 125,823,268 on chromosome 10.

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

Where can I find additional information about UROS?

You and your healthcare professional may find the following resources about UROS 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 UROS gene or gene products?

  • Cosynthase
  • Hydroxymethylbilane hydro-lyase (cyclizing)
  • Uroporphyrinogen co-synthetase
  • Uroporphyrinogen-III cosynthase
  • Uroporphyrinogen III Cosynthetase
  • Uroporphyrinogen-III Synthase
  • uroporphyrinogen III synthase (congenital erythropoietic porphyria)
  • Uroporphyrinogen III Synthetase
  • Uroporphyrinogen Isomerase

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

What glossary definitions help with understanding UROS?

acids ; amino acid ; arginine ; bone marrow ; congenital ; cysteine ; DNA ; enzyme ; gene ; heme ; hemoglobin ; iron ; molecule ; mutation ; oxygen ; protein ; toxic

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


  • Badminton MN, Elder GH. Molecular mechanisms of dominant expression in porphyria. J Inherit Metab Dis. 2005;28(3):277-86. Review. (
  • Desnick RJ, Astrin KH. Congenital erythropoietic porphyria: advances in pathogenesis and treatment. Br J Haematol. 2002 Jun;117(4):779-95. Review. (
  • Elder GH. Genetic defects in the porphyrias: types and significance. Clin Dermatol. 1998 Mar-Apr;16(2):225-33. Review. (
  • Fortian A, Castaño D, Ortega G, Laín A, Pons M, Millet O. Uroporphyrinogen III synthase mutations related to congenital erythropoietic porphyria identify a key helix for protein stability. Biochemistry. 2009 Jan 20;48(2):454-61. doi: 10.1021/bi801731q. (
  • Kauppinen R. Porphyrias. Lancet. 2005 Jan 15-21;365(9455):241-52. Review. (
  • NCBI Gene (
  • Sassa S, Kappas A. Molecular aspects of the inherited porphyrias. J Intern Med. 2000 Feb;247(2):169-78. Review. (
  • Schubert HL, Raux E, Matthews MA, Phillips JD, Wilson KS, Hill CP, Warren MJ. Structural diversity in metal ion chelation and the structure of uroporphyrinogen III synthase. Biochem Soc Trans. 2002 Aug;30(4):595-600. Review. (
  • Shady AA, Colby BR, Cunha LF, Astrin KH, Bishop DF, Desnick RJ. Congenital erythropoietic porphyria: identification and expression of eight novel mutations in the uroporphyrinogen III synthase gene. Br J Haematol. 2002 Jun;117(4):980-7. (
  • Solis C, Aizencang GI, Astrin KH, Bishop DF, Desnick RJ. Uroporphyrinogen III synthase erythroid promoter mutations in adjacent GATA1 and CP2 elements cause congenital erythropoietic porphyria. J Clin Invest. 2001 Mar;107(6):753-62. (
  • To-Figueras J, Badenas C, Mascaró JM, Madrigal I, Merino A, Bastida P, Lecha M, Herrero C. Study of the genotype-phenotype relationship in four cases of congenital erythropoietic porphyria. Blood Cells Mol Dis. 2007 May-Jun;38(3):242-6. Epub 2007 Jan 31. (


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: July 2009
Published: February 8, 2016