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

MT-CYB gene

The MT-CYB gene provides instructions for making cytochrome b, a protein essential for mitochondrial energy production through oxidative phosphorylation.

Normal Function

The MT-CYB gene provides instructions for making a protein called cytochrome b. This protein plays a key role in structures called mitochondria, which convert the energy from food into a form that cells can use. Cytochrome b is one of 11 components of a group of proteins called complex III. In mitochondria, complex III performs one step of a process known as oxidative phosphorylation, in which oxygen and simple sugars are used to create adenosine triphosphate (ATP), the cell's main energy source. During oxidative phosphorylation, the protein complexes, including complex III, drive the production of ATP through a step-by-step transfer of negatively charged particles called electrons. Cytochrome b is involved in the transfer of these particles through complex III.

Although most DNA is packaged in chromosomes within the nucleus (nuclear DNA), mitochondria also have a small amount of their own DNA, called mitochondrial DNA (mtDNA). This type of DNA contains many genes essential for normal mitochondrial function. Cytochrome b is the only component of complex III that is produced from a gene found in mitochondrial DNA.

Health Conditions Related to Genetic Changes

Mitochondrial complex III deficiency

Mutations in the MT-CYB gene can cause mitochondrial complex III deficiency. When caused by mutations in this gene, the condition is usually characterized by muscle weakness (myopathy) and pain, especially during exercise (exercise intolerance). More severely affected individuals can have problems with other body systems, including the liver, kidneys, heart, and brain.

Most MT-CYB gene mutations that cause mitochondrial complex III deficiency change single protein building blocks (amino acids) in the cytochrome b protein or lead to an abnormally short protein. These cytochrome b alterations impair the formation of complex III, severely reducing the complex's activity and oxidative phosphorylation. Researchers believe that impaired oxidative phosphorylation can lead to cell death by reducing the amount of energy available in the cell. It is thought that tissues that require a lot of energy, such as those in the muscles, brain, heart, liver, and kidneys, are most affected by a reduction in oxidative phosphorylation.

The location of the MT-CYB gene in mitochondrial DNA (mtDNA) may help explain why some people have more severe features of the condition than others. Most of the body's cells contain thousands of mitochondria, each with one or more copies of mitochondrial DNA. These cells can have a mix of mitochondria containing mutated and unmutated DNA (heteroplasmy). The severity of mitochondrial complex III deficiency caused by MT-CYB gene mutations is thought to be associated with the percentage of mitochondria with the mitochondrial DNA mutation.

Other Names for This Gene

  • COB
  • CYTB
  • cytochrome b
  • cytochrome b (mitochondrion) [Homo sapiens]
  • MTCYB
  • UQCR3

Frequently Asked Questions

Because the MT-CYB gene is located in mitochondrial DNA, it follows a maternal inheritance pattern. This means the gene is passed only from the mother to all of her children. Fathers do not pass mitochondrial DNA to their offspring. Consequently, if a mother has a pathogenic variant in this gene, all of her children are at risk of inheriting the mutation, although the severity of symptoms can vary significantly among siblings due to a process called heteroplasmy, where different cells may contain different proportions of mutated and normal mitochondrial DNA. What are the specific early warning signs that distinguish MT-CYB-related complex III deficiency from other mitochondrial disorders?::While symptoms overlap with other mitochondrial conditions, MT-CYB mutations often present with prominent exercise intolerance and muscle pain (myalgia) that is disproportionate to the level of activity. Patients may experience rapid fatigue or cramping during physical exertion that does not resolve quickly with rest. In more severe cases, early signs can include unexplained weight loss, failure to thrive in children, or specific organ involvement such as liver dysfunction or cardiac issues, which may appear before significant neurological decline is evident. Can a standard blood test detect MT-CYB mutations, or is a tissue biopsy required?::Diagnosis typically requires genetic testing of mitochondrial DNA. While blood tests can sometimes detect the mutation, they are not always definitive because the proportion of mutated mitochondrial DNA (heteroplasmy level) in blood cells may be too low to detect, or the mutation may be absent in blood but present in affected tissues. Therefore, clinicians often recommend testing muscle tissue or other affected organs via biopsy if blood results are negative but clinical suspicion remains high. Additionally, biochemical tests measuring complex III activity in muscle tissue can support the diagnosis. Is there a specific diet or lifestyle modification that helps manage energy production issues caused by this gene mutation?::There is no specific cure, but management often focuses on optimizing mitochondrial function. Clinicians may recommend a low-intensity, regular exercise routine to maintain muscle strength without triggering severe fatigue, as high-intensity exercise can exacerbate symptoms. Dietary approaches are individualized, but some patients benefit from a diet rich in antioxidants and Coenzyme Q10, which supports the electron transport chain. It is crucial to avoid prolonged fasting, as this can deplete energy reserves and worsen symptoms, so maintaining regular, balanced meals is often advised.
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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