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

CFI gene

The CFI gene provides instructions for making complement factor I, a protein that regulates the immune system to prevent damage to the body's own healthy tissues.

Normal Function

The CFI gene provides instructions for making a protein called complement factor I. This protein helps regulate the complement system, a part of the immune response that destroys foreign invaders, triggers inflammation, and removes debris. By preventing the complement system from activating when it is not needed, complement factor I protects the body's healthy cells from being attacked.

Health Conditions Related to Genetic Changes

Complement factor I deficiency

Mutations in the CFI gene can lead to complement factor I deficiency, a disorder characterized by immune system dysfunction. The lack of functional protein allows for uncontrolled activation of the complement system, which decreases levels of the complement protein C3. This reduces the body's ability to fight infections and may lead to autoimmune disorders where the immune system attacks its own tissues.

Other disorders

Mutations in the CFI gene have been identified in people with glomerulonephritis with isolated C3 deposits. This condition involves serious kidney malfunction caused by an overactive complement system attacking kidney cells, leading to proteinuria (loss of protein in the urine).

Additionally, a common variation in the CFI gene has been associated with age-related macular degeneration (AMD), a leading cause of vision loss in older adults. Researchers suggest this variation may alter how the gene is expressed, though the exact relationship to AMD development remains under study.

Other Names for This Gene

  • AHUS3
  • C3b-INA
  • C3b-inactivator
  • C3B/C4B inactivator
  • C3BINA
  • CFAI_HUMAN
  • complement component I
  • complement control protein factor I
  • complement factor I heavy chain
  • complement factor I preproprotein
  • FI
  • IF
  • KAF
  • Konglutinogen-activating factor
  • light chain of factor I

Genomic Location

The CFI gene is found on chromosome 4.

Frequently Asked Questions

Diagnosis typically involves measuring serum complement levels, especially low C3 with normal C4, and performing a functional assay for factor I activity. Genetic sequencing of the CFI gene confirms pathogenic variants.
Good hygiene, up‑to‑date vaccinations (including pneumococcal, meningococcal, and Haemophilus influenzae type b), and prompt treatment of infections are essential. Some clinicians recommend prophylactic antibiotics during high‑risk periods, but this should be individualized.
Patients may experience recurrent bacterial infections (especially encapsulated organisms), persistent low C3 levels, signs of autoimmune disease such as rash or joint pain, and kidney manifestations like proteinuria or hematuria indicating glomerulonephritis.
Plasma or purified factor I replacement is not widely available, but complement inhibitors such as eculizumab (a C5 blocker) have been used off‑label for severe kidney disease or atypical hemolytic uremic syndrome linked to CFI mutations. Clinical trials are ongoing for newer agents that more directly restore factor I function.
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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