The HMBS gene helps your body make heme, a substance needed for hemoglobin and for many liver enzymes. If the HMBS gene does not work well, certain natural chemicals can build up and may trigger attacks of severe belly pain, nausea, vomiting, weakness, or nerve symptoms. Pathogenic variants in the HMBS gene can cause acute intermittent porphyria, a condition that often has symptom-free periods between attacks. Many people carrying an HMBS variant never develop symptoms.
What is a gene?
Genes are stretches of DNA that carry instructions your cells use to make molecules or regulate processes in the body. Some genes provide the directions for producing proteins, the cell parts that do much of the work. Others mainly control the timing, location, and level of activity of certain processes. Taken together, genes help shape how the body develops and works, and they can influence how you respond to food, stress, or medications. To explore this topic further, you can learn more in the LearnHub.
Overview
HMBSGene function
What if the gene works
The HMBS gene helps your body make heme, a part of hemoglobin that carries oxygen in red blood cells. Its protein also supports other heme proteins that help cells use oxygen and make energy.
Risk
What if the gene does not work
Milder and more severe variants in the HMBS gene can lower activity of a heme-making enzyme and cause acute intermittent porphyria. Effects range from no symptoms to repeated attacks of severe belly pain, nausea, weakness, and nerve or mood symptoms.
Treatment
What does that mean for your treatment
A pathogenic variant in the HMBS gene supports porphyria-specific treatment planning. Acute attacks are commonly treated with intravenous hemin, and frequent attacks may be reduced with givosiran. Drug choices also matter, because some medicines can trigger attacks and are usually avoided.
Conditions associated with HMBS
Gene function
What if the gene works
Your body uses the HMBS gene to help make heme, a small but vital part of proteins that carry oxygen and support energy production. When the HMBS gene is active, cells can move through an early step in the heme making pathway in an orderly way. This helps blood, muscles, and many organs do routine work each day.
The HMBS gene helps cells join small building blocks into a longer chain during heme production. This is an early, essential step in how the HMBS gene helps make heme.
By helping make heme, this gene supports hemoglobin in red blood cells and myoglobin in muscle. These proteins help carry oxygen through the body and hold oxygen where muscles need it.
Heme is also used by proteins that help cells turn food and oxygen into usable energy. This means HMBS supports steady day to day cell activity in many tissues.
Many body enzymes need heme to do their jobs properly. Through its protein, the HMBS gene helps supply this key part so routine cell chemistry can run smoothly.
What is a variant?
A genetic variant is a minor difference in the DNA sequence within a gene. These differences are common and are part of normal human variation, including differences in appearance, metabolism, and the way the body responds to environmental factors. Many variants do not have any noticeable effect and are simply part of natural genetic diversity. At times, though, a variant changes how a gene works to a greater extent and may contribute to, or directly cause, a genetic condition.
Genetic variants
What if the gene does not work
Variants in the HMBS gene can change the enzyme’s shape, stability, or how much of it a cell makes, which explains how variants influence the HMBS gene's function. Some variants leave partial activity, while more severe variants, including truncating or splice changes, can lead to very little working enzyme. In acute intermittent porphyria, lower residual activity may be linked to a greater chance of earlier or more noticeable features, although this varies from person to person.
How variants are linked to conditions
Pathogenic variants in the HMBS gene cause acute intermittent porphyria, and the link between the HMBS gene and acute intermittent porphyria is well established. A confirmed disease-causing variant can support or confirm this diagnosis, although symptoms vary widely and many carriers never have attacks. Rarely, people with harmful variants in both copies of the HMBS gene can have a much earlier-onset and more severe porphyria.
A variant is not necessarily a diagnosis
Genetic variants are common across the general population and are a routine part of human biology. Most do not directly affect health and simply contribute to normal human diversity. For many genes, having a variant associated with a condition points to a possible link rather than a certain diagnosis, particularly when several genes and environmental influences also play a role. In other situations, though, one well-studied variant in a particular gene is enough to confirm a diagnosis.
Are there things that can increase or change my risk?
For people with an HMBS gene change, the biggest shifts in risk come from common attack triggers rather than from the gene changing over time. Risk can rise with certain medicines, fasting or crash dieting, alcohol, smoking, hormone changes such as around periods, illness, and major stress. Avoiding known triggers can help lower the chance of attacks.
Inheritance of HMBS variants
Most disease-causing variants in the HMBS gene are inherited in an autosomal dominant pattern, which is the usual HMBS gene inheritance pattern seen in acute intermittent porphyria. This means a person can develop the condition with one altered copy of the gene. If a parent carries an HMBS variant linked to the condition, each child has a 50% chance of inheriting that same variant. Sons and daughters are affected at similar rates because the HMBS gene is on chromosome 11, not on a sex chromosome.
What does my HMBS result mean for my family?
For HMBS, family impact is usually discussed in the setting of acute intermittent porphyria, which often runs in families in a dominant pattern. That means close relatives such as parents, brothers, sisters, and children may also carry the same gene change, even if they have never had symptoms. In families with a confirmed HMBS change, relatives may want to talk with their care team about whether testing would clarify their own risk.
Genetic information
Genetic testing
How you find out about variants
Genetic testing for the HMBS gene usually uses a blood or saliva sample to read the gene and look for variants linked to acute intermittent porphyria. This is how you find out about HMBS variants in a person with suggestive symptoms or in relatives of someone with a known family variant.
When should I consider getting a genetic test?
Consider HMBS testing if you or close relatives have acute intermittent porphyria, a condition that can cause repeated attacks of severe stomach pain, vomiting, weakness, or dark urine. It can also help clarify the cause after abnormal urine tests during an attack.
A genetic result is only one part of your health story
Genetic findings can be informative, but how much they reveal varies by the condition and the specific type of variant. Over time, factors such as age, environment, lifestyle, and medical history also shape how the body works. For some conditions, especially those linked to many genes, genetic information offers context rather than certainty and is most useful when viewed alongside other parts of care. In other situations, a genetic result can provide a clear answer. In medication response, it can also help put personal experiences into context.
Pharmacogenetics
How variants shape your response to medicine
Genetic variation can affect how people respond to medicines. This helps explain why the same treatment works well for one person, seems weaker in another, or causes more side effects in someone else. Part of the reason is that inherited differences can change how the body breaks down a drug, moves it around, or responds to it at its target. These differences are common and can influence both how well a medicine works and how sensitive a person may be to it.
What is Pharmacogenetics?
Pharmacogenetics studies how genetic differences affect the response to medications. It examines relationships between genes and drugs to help explain why treatments do not work the same way for everyone. With many medications, genes influence either how strongly a drug works or how the body processes it. This information can support more informed, individualized treatment decisions. You can read more about this topic in the LearnHub.
Drugs influenced by the HMBS gene
When should I seek medical advice?
There is not yet a broadly confirmed role for this gene in guiding medication choices, so general health or medicine concerns are best discussed with your doctor. HMBS is different in one important way: in people living with an HMBS change that causes acute intermittent porphyria, some medicines can trigger attacks, so it is worth checking with your doctor or pharmacist before starting a new prescription.
History
Acute intermittent porphyria, the condition most closely linked to the HMBS gene, was recognized from recurring attacks long before the gene itself was identified. Initially understood only through visible traits, later recognized as tied to a specific gene, this history reflects the broader shift from bedside observation to modern genetics. In the 20th century, researchers connected the condition to reduced activity of an enzyme then called porphobilinogen deaminase, which is encoded by the HMBS gene. Building on this early work, scientists mapped the HMBS gene to chromosome 11 and began identifying inherited variants in affected families. Over time, descriptions became more precise as DNA testing replaced older biochemical clues alone. This history helped turn a puzzling inherited illness into a clearly defined genetic diagnosis.