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

X-linked severe combined immunodeficiency

(often shortened to X-linked SCID)
Reviewed May 2009

What is X-linked SCID?

X-linked severe combined immunodeficiency (SCID) is an inherited disorder of the immune system that occurs almost exclusively in males. Boys with X-linked SCID are prone to recurrent and persistent infections caused by certain bacteria, viruses, and fungi. The organisms that cause these infections are described as opportunistic because they ordinarily do not cause illness in healthy people. However, in people with X-linked SCID they cause very serious or life-threatening illnesses. Many infants with X-linked SCID experience chronic diarrhea and skin rashes, and grow more slowly than other children. Without treatment, affected males usually do not live beyond infancy.

How common is X-linked SCID?

X-linked SCID is the most common form of severe combined immunodeficiency. Its exact incidence is unknown, but the condition probably affects at least 1 in 50,000 to 100,000 newborns.

What genes are related to X-linked SCID?

Mutations in the IL2RG gene cause X-linked SCID. The IL2RG gene provides instructions for making a protein that is critical for normal immune system function. This protein is necessary for the growth and maturation of developing immune system cells called lymphocytes. Lymphocytes defend the body against potentially harmful invaders, make antibodies, and help regulate the entire immune system. Mutations in the IL2RG gene prevent these cells from developing and functioning normally. Without functional lymphocytes, the body is unable to fight off infections.

Related Gene(s)

Changes in this gene are associated with X-linked severe combined immunodeficiency.

  • IL2RG

How do people inherit X-linked SCID?

This condition is inherited in an X-linked recessive pattern. The gene associated with this condition is located on the X chromosome, which is one of the two sex chromosomes. In males (who have only one X chromosome), one altered copy of the gene in each cell is sufficient to cause the condition. In females (who have two X chromosomes), a mutation would have to occur in both copies of the gene to cause the disorder. Because it is unlikely that females will have two altered copies of this gene, males are affected by X-linked recessive disorders much more frequently than females. A characteristic of X-linked inheritance is that fathers cannot pass X-linked traits to their sons.

Where can I find information about diagnosis or management of X-linked SCID?

These resources address the diagnosis or management of X-linked SCID and may include treatment providers.

  • Baby's First Test: Severe Combined Immunodeficiency (
  • Gene Review: X-Linked Severe Combined Immunodeficiency (
  • Genetic Testing Registry: X-linked severe combined immunodeficiency (
  • MedlinePlus Encyclopedia: Immunodeficiency Disorders (
  • National Marrow Donor Program: Severe Combined Immunodeficiency and Transplant (

You might also find information on the diagnosis or management of X-linked SCID in Educational resources and Patient support.

General information about the diagnosis ( and management ( of genetic conditions is available in the Handbook. Read more about genetic testing (, particularly the difference between clinical tests and research tests (

To locate a healthcare provider, see How can I find a genetics professional in my area? ( in the Handbook.

Where can I find additional information about X-linked SCID?

You may find the following resources about X-linked SCID helpful. These materials are written for the general public.

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

What other names do people use for X-linked SCID?

  • SCIDX1
  • X-SCID

For more information about naming genetic conditions, see the Genetics Home Reference Condition Naming Guidelines ( and How are genetic conditions and genes named? ( in the Handbook.

What if I still have specific questions about X-linked SCID?

Ask the Genetic and Rare Diseases Information Center (

What glossary definitions help with understanding X-linked SCID?

bacteria ; cell ; chromosome ; chronic ; failure to thrive ; gene ; immune system ; immunodeficiency ; incidence ; inheritance ; inherited ; mutation ; newborn screening ; protein ; recessive ; screening ; sex chromosomes ; X-linked recessive

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


  • Buckley RH. Molecular defects in human severe combined immunodeficiency and approaches to immune reconstitution. Annu Rev Immunol. 2004;22:625-55. Review. (
  • Chinen J, Puck JM. Successes and risks of gene therapy in primary immunodeficiencies. J Allergy Clin Immunol. 2004 Apr;113(4):595-603; quiz 604. Review. (
  • Church AC. X-linked severe combined immunodeficiency. Hosp Med. 2002 Nov;63(11):676-80. Review. (
  • Gene Review: X-Linked Severe Combined Immunodeficiency (
  • Gennery AR, Cant AJ. Diagnosis of severe combined immunodeficiency. J Clin Pathol. 2001 Mar;54(3):191-5. Review. (
  • Huang H, Manton KG. Newborn screening for severe combined immunodeficiency (SCID): a review. Front Biosci. 2005 May 1;10:1024-39. Review. (
  • Puck JM, Malech HL. Gene therapy for immune disorders: good news tempered by bad news. J Allergy Clin Immunol. 2006 Apr;117(4):865-9. 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: May 2009
Published: February 1, 2016