Skip navigation

Official websites use .gov
A .gov website belongs to an official government organization in the United States.

Secure .gov websites use HTTPS
A lock ( ) or https:// means you’ve safely connected to the .gov website. Share sensitive information only on official, secure websites.

URL of this page:

HAL gene

histidine ammonia-lyase

Normal Function

The HAL gene provides instructions for making an enzyme called histidase. Histidase breaks down the amino acid histidine, a building block of most proteins. Histidase is active (expressed) primarily in the liver and the skin. This enzyme breaks down histidine to a molecule called urocanic acid. In the liver, urocanic acid is broken down to form another amino acid called glutamic acid. In the skin, urocanic acid is involved in the response to ultraviolet (UV) light.

Health Conditions Related to Genetic Changes


At least four mutations in the HAL gene have been found to cause histidinemia. All of these mutations change single amino acids in the histidase enzyme. These mutations are thought to decrease or eliminate enzyme activity, resulting in an inability to break down histidine. Histidine that is not broken down accumulates in the blood, but it typically causes no health problems.

More About This Health Condition

Other Names for This Gene

  • HIS
  • histidase
  • HSTD

Additional Information & Resources

Tests Listed in the Genetic Testing Registry

Scientific Articles on PubMed

Catalog of Genes and Diseases from OMIM

Gene and Variant Databases


  • Eckhart L, Schmidt M, Mildner M, Mlitz V, Abtin A, Ballaun C, Fischer H, Mrass P, Tschachler E. Histidase expression in human epidermal keratinocytes: regulation by differentiation status and all-trans retinoic acid. J Dermatol Sci. 2008 Jun;50(3):209-15. doi: 10.1016/j.jdermsci.2007.12.009. Epub 2008 Feb 15. Citation on PubMed
  • Kawai Y, Moriyama A, Asai K, Coleman-Campbell CM, Sumi S, Morishita H, Suchi M. Molecular characterization of histidinemia: identification of four missense mutations in the histidase gene. Hum Genet. 2005 Apr;116(5):340-6. doi: 10.1007/s00439-004-1232-5. Epub 2005 Jan 27. Erratum In: Hum Genet. 2005 Dec;118(3-4):531-2. Citation on PubMed
  • Suchi M, Sano H, Mizuno H, Wada Y. Molecular cloning and structural characterization of the human histidase gene (HAL). Genomics. 1995 Sep 1;29(1):98-104. doi: 10.1006/geno.1995.1219. Citation on PubMed

The information on this site should not be used as a substitute for professional medical care or advice. Contact a health care provider if you have questions about your health.