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CFI gene

complement factor I

Normal Function

The CFI gene provides instructions for making a protein called complement factor I. This protein helps regulate a part of the body's immune response known as the complement system. The complement system is a group of proteins that work together to destroy foreign invaders (such as bacteria and viruses), trigger inflammation, and remove debris from cells and tissues. This system must be carefully regulated so it targets only unwanted materials and does not attack the body's healthy cells. Complement factor I and several related proteins protect healthy cells by preventing activation of the complement system when it is not needed.

Health Conditions Related to Genetic Changes

Complement factor I deficiency

At least 10 mutations in the CFI gene have been identified in people with complement factor I deficiency, a disorder characterized by immune system dysfunction. The mutations result in abnormal, nonfunctional, or absent complement factor I.

The lack (deficiency) of functional complement factor I protein allows uncontrolled activation of the complement system. The unregulated activity of the complement system decreases blood levels of another complement protein called C3, reducing the immune system's ability to fight infections. In addition, the immune system may malfunction and attack its own tissues, resulting in autoimmune disorders.

More About This Health Condition

Age-related macular degeneration

MedlinePlus Genetics provides information about Age-related macular degeneration

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Atypical hemolytic-uremic syndrome

MedlinePlus Genetics provides information about Atypical hemolytic-uremic syndrome

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C3 glomerulopathy

MedlinePlus Genetics provides information about C3 glomerulopathy

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Other disorders

Mutations in the CFI gene have also been found in people with glomerulonephritis with isolated C3 deposits. This condition, which may also occur in people with complement factor I deficiency, is characterized by kidney malfunction that can be serious or life-threatening. The CFI gene mutations identified in this disorder result in an abnormal or nonfunctional version of complement factor I. The defective protein allows uncontrolled activation of the complement system. The overactive complement system attacks certain kidney cells, which damages the kidneys and leads to a loss of protein in the urine (proteinuria).

A common variation (polymorphism) in the CFI gene has also been associated with age-related macular degeneration (AMD). AMD is a leading cause of vision loss among older adults. It is characterized by damage to the retina and a loss of sharp vision (visual acuity). Researchers suggest that the CFI gene variation that has been associated with AMD changes the way the gene is activated (expressed). It is unclear how this change is related to the development of AMD. A combination of genetic and environmental factors likely determines the risk of developing this complex eye disorder.

Other Names for This Gene

  • AHUS3
  • C3b-INA
  • C3b-inactivator
  • C3B/C4B inactivator
  • C3BINA
  • 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

Additional Information & Resources

Tests Listed in the Genetic Testing Registry

Scientific Articles on PubMed

Catalog of Genes and Diseases from OMIM

Gene and Variant Databases


  • Baracho GV, Nudelman V, Isaac L. Molecular characterization of homozygous hereditary factor I deficiency. Clin Exp Immunol. 2003 Feb;131(2):280-6. doi: 10.1046/j.1365-2249.2003.02077.x. Citation on PubMed or Free article on PubMed Central
  • Fagerness JA, Maller JB, Neale BM, Reynolds RC, Daly MJ, Seddon JM. Variation near complement factor I is associated with risk of advanced AMD. Eur J Hum Genet. 2009 Jan;17(1):100-4. doi: 10.1038/ejhg.2008.140. Epub 2008 Aug 6. Citation on PubMed or Free article on PubMed Central
  • Kavanagh D, Richards A, Noris M, Hauhart R, Liszewski MK, Karpman D, Goodship JA, Fremeaux-Bacchi V, Remuzzi G, Goodship TH, Atkinson JP. Characterization of mutations in complement factor I (CFI) associated with hemolytic uremic syndrome. Mol Immunol. 2008 Jan;45(1):95-105. doi: 10.1016/j.molimm.2007.05.004. Epub 2007 Jun 26. Citation on PubMed
  • Nilsson SC, Trouw LA, Renault N, Miteva MA, Genel F, Zelazko M, Marquart H, Muller K, Sjoholm AG, Truedsson L, Villoutreix BO, Blom AM. Genetic, molecular and functional analyses of complement factor I deficiency. Eur J Immunol. 2009 Jan;39(1):310-23. doi: 10.1002/eji.200838702. Citation on PubMed
  • Ponce-Castro IM, Gonzalez-Rubio C, Delgado-Cervino EM, Abarrategui-Garrido C, Fontan G, Sanchez-Corral P, Lopez-Trascasa M. Molecular characterization of Complement Factor I deficiency in two Spanish families. Mol Immunol. 2008 May;45(10):2764-71. doi: 10.1016/j.molimm.2008.02.008. Epub 2008 Mar 28. Citation on PubMed
  • Richard I. The genetic and molecular bases of monogenic disorders affecting proteolytic systems. J Med Genet. 2005 Jul;42(7):529-39. doi: 10.1136/jmg.2004.028118. Citation on PubMed or Free article on PubMed Central
  • Servais A, Fremeaux-Bacchi V, Lequintrec M, Salomon R, Blouin J, Knebelmann B, Grunfeld JP, Lesavre P, Noel LH, Fakhouri F. Primary glomerulonephritis with isolated C3 deposits: a new entity which shares common genetic risk factors with haemolytic uraemic syndrome. J Med Genet. 2007 Mar;44(3):193-9. doi: 10.1136/jmg.2006.045328. Epub 2006 Oct 3. Citation on PubMed or Free article on PubMed Central
  • Vyse TJ, Morley BJ, Bartok I, Theodoridis EL, Davies KA, Webster AD, Walport MJ. The molecular basis of hereditary complement factor I deficiency. J Clin Invest. 1996 Feb 15;97(4):925-33. doi: 10.1172/JCI118515. Citation on PubMed or Free article on PubMed Central
  • Vyse TJ, Spath PJ, Davies KA, Morley BJ, Philippe P, Athanassiou P, Giles CM, Walport MJ. Hereditary complement factor I deficiency. QJM. 1994 Jul;87(7):385-401. Citation on PubMed

The information on this site should not be used as a substitute for professional medical care or advice. Contact a health care provider if you have questions about your health.