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

HMBS gene

The HMBS gene provides instructions for making an enzyme essential for the production of heme, a vital component of hemoglobin and other proteins.

Normal Function

The HMBS gene provides instructions for making an enzyme known as hydroxymethylbilane 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. Hydroxymethylbilane synthase is responsible for the third step in this process, which combines four molecules of porphobilinogen to form a compound called hydroxymethylbilane. In subsequent steps, five other enzymes produce and modify compounds that ultimately lead to heme.

Health Conditions Related to Genetic Changes

Porphyria

More than 300 mutations in the HMBS gene have been identified in people with a form of porphyria known as acute intermittent porphyria. Some of these mutations change single protein building blocks (amino acids) in hydroxymethylbilane synthase. Other mutations add or delete genetic material within the HMBS gene, which alters the structure and function of the enzyme.

Mutations in the HMBS gene reduce the activity of hydroxymethylbilane synthase, allowing compounds called porphyrins to build up in the liver and other organs. These compounds are formed during the normal process of heme production, but reduced activity of hydroxymethylbilane synthase allows them to accumulate to toxic levels. This buildup, in combination with nongenetic factors such as certain drugs, alcohol, smoking, and dieting, leads to attacks of severe abdominal pain and other symptoms in people with acute intermittent porphyria.

Other Names for This Gene

  • HEM3_HUMAN
  • Hydroxymethylbilane Synthetase
  • PBG-D
  • PBGD
  • Porphobilinogen Ammonia-Lyase
  • Porphobilinogen ammonia-lyase (polymerizing)
  • Porphobilinogen Deaminase
  • Porphyrinogen Synthetase
  • Pre-uroporphyrinogen synthase
  • Preuroporphyrinogen Synthetase
  • UPS
  • Uroporphyrinogen synthase

Genomic Location

The HMBS gene is found on chromosome 11.

Frequently Asked Questions

Common triggers include barbiturates, certain antibiotics (e.g., sulfonamides), anti‑seizure drugs, hormonal contraceptives, alcohol, smoking, and fasting or extreme dieting. Always check drug safety lists for “porphyria‑safe” alternatives and maintain a regular, balanced diet with adequate carbohydrate intake.
Pregnancy can increase the risk of AIP attacks because of hormonal changes and altered metabolism. Women with a known HMBS mutation should be managed by a multidisciplinary team, avoid known triggers, and have access to intravenous hemin for emergencies. The fetus inherits a 50% chance of the mutation, but most carriers are asymptomatic; prenatal testing is optional and should be discussed with a genetic counselor.
Mutations in HMBS reduce the activity of hydroxymethylbilane synthase, a key enzyme in heme synthesis. When the enzyme works poorly, toxic porphyrin precursors build up in the liver and trigger the nervous system, leading to severe abdominal pain, nausea, and neurological symptoms during an AIP attack.
Yes. AIP is autosomal dominant with low penetrance, meaning many carriers never have symptoms but can still pass the mutation to children. Testing allows you to avoid triggers, receive education, and consider prophylactic measures such as hemin infusions during high‑risk periods (e.g., surgery, pregnancy).
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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