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HAL gene
Reviewed by Medicinsk Expert•Last updated: 2026-10-22•Gene

HAL gene

The HAL gene provides instructions for making the enzyme histidase, which is essential for breaking down the amino acid histidine.

Overview

The HAL gene provides instructions for making an enzyme called histidase. This enzyme is primarily active in the liver and skin. Its main function is to break down histidine, an amino acid that serves as a building block for proteins.

In the liver, histidase converts histidine into urocanic acid, which is eventually broken down into glutamate. In the skin, urocanic acid plays a role in the body's response to ultraviolet (UV) light.

Health Conditions Related to Genetic Changes

Histidinemia

Variants in the HAL gene can cause an inherited condition known as histidinemia. This condition is characterized by high levels of histidine in the blood, urine, and cerebrospinal fluid. The effects of this condition include:

  • Decreased or eliminated histidase activity, which prevents the body from breaking down histidine effectively.
  • In rare cases, potential intellectual or learning disabilities and behavioral complications.
  • An increased risk of developmental problems if medical complications occur during or shortly after birth.

Frequently Asked Questions

Newborns with histidinemia may show feeding difficulties, poor weight gain, or mild jaundice. In some cases, they may have a slightly elevated blood pressure or a subtle developmental delay that becomes apparent within the first few months. Parents should watch for persistent vomiting or lethargy and report these symptoms to a pediatrician promptly.
Diagnosis begins with a blood test that measures plasma histidine levels; values above the normal range (typically > 200 µmol/L) raise suspicion. A confirmatory test is a urine amino acid analysis showing elevated histidine and a cerebrospinal fluid study if neurological symptoms are present. Genetic testing for pathogenic variants in the HAL gene is the definitive diagnostic step.
Yes, a low‑histidine diet can reduce plasma histidine levels and lessen symptoms. Foods high in histidine—such as red meat, poultry, fish, dairy products, soy, nuts, and certain grains—should be limited. A registered dietitian can design a balanced plan that maintains overall nutrition while keeping histidine intake within safe limits.
Children with untreated or poorly controlled histidinemia may develop mild intellectual or learning disabilities, speech delays, or behavioral issues. Regular developmental screening at 6‑month intervals, neuropsychological testing, and periodic hearing and vision evaluations are recommended to detect and address any emerging deficits early.
Source: MedlinePlus, National Library of Medicine

MedlinePlus brings together authoritative health information from the National Library of Medicine (NLM), the National Institutes of Health (NIH), and other government agencies and health-related organizations.

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