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Genetics Home Reference: your guide to understanding genetic conditions     A service of the U.S. National Library of Medicine®


Reviewed September 2010

What is the official name of the CFI gene?

The official name of this gene is “complement factor I.”

CFI is the gene's official symbol. The CFI gene is also known by other names, listed below.

What is the normal function of the CFI gene?

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.

Does the CFI gene share characteristics with other genes?

The CFI gene belongs to a family of genes called complement (complement system).

A gene family is a group of genes that share important characteristics. Classifying individual genes into families helps researchers describe how genes are related to each other. For more information, see What are gene families? ( in the Handbook.

How are changes in the CFI gene related to health conditions?

complement factor I deficiency - caused by mutations in the CFI gene

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.

other disorders - associated with the CFI gene

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 the elderly. 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.

Where is the CFI gene located?

Cytogenetic Location: 4q25

Molecular Location on chromosome 4: base pairs 109,731,221 to 109,802,225

(Homo sapiens Annotation Release 107, GRCh38.p2) (NCBI (

The CFI gene is located on the long (q) arm of chromosome 4 at position 25.

The CFI gene is located on the long (q) arm of chromosome 4 at position 25.

More precisely, the CFI gene is located from base pair 109,731,221 to base pair 109,802,225 on chromosome 4.

See How do geneticists indicate the location of a gene? ( in the Handbook.

Where can I find additional information about CFI?

You and your healthcare professional may find the following resources about CFI helpful.

You may also be interested in these resources, which are designed for genetics professionals and researchers.

What other names do people use for the CFI gene or gene products?

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

See How are genetic conditions and genes named? ( in the Handbook.

What glossary definitions help with understanding CFI?

atypical ; autoimmune ; bacteria ; complement protein ; deficiency ; expressed ; factor I ; gene ; immune response ; immune system ; inflammation ; kidney ; polymorphism ; protein ; proteinuria ; retina ; syndrome ; visual acuity

You may find definitions for these and many other terms in the Genetics Home Reference Glossary.


  • Baracho GV, Nudelman V, Isaac L. Molecular characterization of homozygous hereditary factor I deficiency. Clin Exp Immunol. 2003 Feb;131(2):280-6. (
  • 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. (
  • 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. Epub 2007 Jun 26. (
  • NCBI Gene (
  • Nilsson SC, Trouw LA, Renault N, Miteva MA, Genel F, Zelazko M, Marquart H, Muller K, Sjöholm 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. (
  • Ponce-Castro IM, González-Rubio C, Delgado-Cerviño EM, Abarrategui-Garrido C, Fontán G, Sánchez-Corral P, López-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. (
  • Richard I. The genetic and molecular bases of monogenic disorders affecting proteolytic systems. J Med Genet. 2005 Jul;42(7):529-39. Review. (
  • Servais A, Frémeaux-Bacchi V, Lequintrec M, Salomon R, Blouin J, Knebelmann B, Grünfeld JP, Lesavre P, Noël 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. Epub 2006 Oct 3. (
  • 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. (
  • Vyse TJ, Späth 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. Review. (


The resources on this site should not be used as a substitute for professional medical care or advice. Users seeking information about a personal genetic disease, syndrome, or condition should consult with a qualified healthcare professional. See How can I find a genetics professional in my area? ( in the Handbook.

Reviewed: September 2010
Published: February 8, 2016