Your browser does not support javascript:   Search for gard hereSearch for news-and-events here.


Genetic and Rare Diseases Information Center (GARD)

Klinefelter syndrome

Other Names for this Disease
  • Klinefelter's syndrome
  • XXY syndrome
Related Diseases
See Disclaimer regarding information on this site. Some links on this page may take you to organizations outside of the National Institutes of Health.

Your Question

Klinefelter syndrome can be caused by nondisjunction. What increases the risk of males or females to have nondisjunction during meiosis? What research is currently underway for Klinefelter syndrome?

Our Answer

We have identified the following information that we hope you find helpful. If you still have questions, please contact us.

What is Klinefelter syndrome?

Klinefelter syndrome (KS) is a condition that may be present in an individual that has two X chromosomes and one Y chromosome (47, XXY); usually, males have one X and one Y (XY) and females have two X chromosomes (XX). Some individuals with a 47, XXY chromosome finding may have no obvious signs or symptoms of KS while others may have several features and varying degrees of cognitive, social, behavioral, and learning difficulties.[1] Because features may not be apparent until mid to late adolescence, the term “Klinefelter syndrome” is often reserved for affected adolescents and adults. Although the vast majority of boys with KS identify as males, some individuals develop atypical gender identities.[2] In adulthood, individuals with Klinefelter syndrome may have primary hypogonadism (decreased testosterone production), small testes, enlarged breast tissue (gynecomastia), tall stature, and/or other features.[1] The vast majority of males with KS are infertile, but many produce sperm and may be able to conceive with assisted reproduction. Treatment varies among individuals and may include testosterone therapy; however, this therapy may not be appropriate for all individuals.[2]
Last updated: 9/20/2013

What is nondisjunction and how does it result in Klinefelter syndrome?

Nondisjunction is an error in cell division. The cells destined to become sperm and eggs undergo a process known as meiosis. In this process, the 46 chromosomes in the cell separate, ultimately producing two new cells having 23 chromosomes each. Before meiosis is completed, however, chromosomes pair with their corresponding chromosomes and exchange bits of genetic material. In women, X chromosomes pair; in men, the X and Y chromosome pair. After the exchange, the chromosomes separate, and meiosis continues.[3]

In some cases, the Xs or the X chromosome and Y chromosome fail to pair and fail to exchange genetic material. Occasionally, this results in their moving independently to the same cell, producing either an egg with two Xs, or a sperm having both an X and a Y chromosome. When a sperm having both an X and a Y chromosome fertilizes an egg having a single X chromosome, or a normal Y- bearing sperm fertilizes an egg having two X chromosomes, an XXY male is conceived.[3]

Last updated: 12/10/2008

What causes nondisjunction?

The cause of nondisjunction is unknown.  Nondisjunction seems to be a chance event.  Nothing that an individual does or doesn't do during their reproductive years can cause these chromosomal changes.  We do know that nondisjunction occurs more frequently in the eggs of women as they get older.[4]
Last updated: 12/10/2008

How can I learn about research involving Klinefelter syndrome?

The U.S. National Institutes of Health, through the National Library of Medicine, developed to provide patients, family members, and members of the public with current information on clinical research studies. You can find clinical trials for individuals with Klinefelter syndrome by clicking on the above link. Check this site often for regular updates.

You can also contact the Patient Recruitment and Public Liaison (PRPL) Office at the National Institutes of Health (NIH). We recommend calling the toll-free number listed below to speak with a specialist, who can help you determine if you are eligible for any clinical trials.   If you are located outside the United States, and would like to be contacted via telephone, you will need to provide your telephone number in full, including area code and international dialing prefix.

Patient Recruitment and Public Liaison Office
NIH Clinical Center
Bethesda, Maryland 20892-2655
Toll-free: 800-411-1222
Fax: 301-480-9793
Web site:

You can find helpful general information on clinical trials at the following Web page.

A tutorial about clinical trials that can also help answer your questions can be found at the following link from the National Library of Medicine:

Last updated: 9/18/2013

Other Names for this Disease
  • Klinefelter's syndrome
  • XXY syndrome
Related Diseases
See Disclaimer regarding information on this site. Some links on this page may take you to organizations outside of the National Institutes of Health.