Genetics Northern California

Klinefelter Syndrome

What is Klinefelter syndrome (KS)?
How common is Klinefelter syndrome?
What causes Klinefelter syndrome?
How is Klinefelter syndrome diagnosed?
Can Klinefelter syndrome be diagnosed during pregnancy?
How is Klinefelter syndrome treated?
Where can I get more information about Klinefelter syndrome?

What is Klinefelter syndrome?
Klinefelter syndrome (KS) is a genetic condition in males that can include learning and behavior challenges, minor physical features, and infertility. Individuals with KS are often taller than expected for their age and family background and can have lower muscle tone. However, the features of this condition are variable and some individuals with KS do not have any obvious signs or symptoms.

Medical concerns in KS
Low testosterone and infertility are common in KS. Puberty may be delayed. There is also a higher rate of breast cancer, thyroid problems, autoimmune diseases, and osteoporosis in men with this condition.

Learning and behavior in KS
There is a wide range of learning abilities in individuals with KS. Many children with KS have delays in learning and speech, but IQ scores usually fall in the average range or slightly below the average range. Behavior concerns may include shyness, attention problems, delayed maturity, autism, and difficulty with social skills. 

How common is KS?
It is estimated that one in every 600 newborn boys has KS.

47,XXYWhat causes KS?
Klinefelter syndrome is caused by an extra chromosome. Individuals with KS have two X chromosomes and one Y chromosome, instead of the usual pair of male sex chromosomes (one X and one Y). This chromosome arrangement is called 47,XXY, and is another way to refer to KS. KS happens as a random event when an egg or sperm is being formed and is not inherited.

How is KS diagnosed?
The diagnosis is usually made by a blood test that shows an extra X chromosome in a male. KS is rarely diagnosed at birth since babies with this condition look like other babies. Chromosome testing may be offered for children with learning delays, teen boys with puberty issues, or men with infertility.  

Can KS be diagnosed during pregnancy?
KS can be diagnosed during pregnancy by testing a DNA sample from the developing baby. The sample can be obtained either by a chorionic villus sampling (CVS) or amniocentesis procedure.

One prenatal screening test, NIPT, can identify pregnancies that have a higher chance for KS. More testing is needed to confirm the diagnosis.

How is KS treated?
Health care for individuals with KS is based on symptoms and when the diagnosis is made. Children may benefit from early intervention programs to support learning. Treatment for adolescents and adults may include testosterone replacement. For individuals with KS who want to have children, fertility clinics offer a variety of options.

Where can I get more information about Klinefelter syndrome?
Genetics Home Reference: Klinefelter syndrome - General information about KS provided as a service of the U.S. National Library of Health. Links to many related sites are included on the page.
The Association for X and Y Chromosome Variations (AXYS) – National support organization for those affected by one or more extra sex chromosome. 

Last updated: January 10, 2018
Reviewed by: Kimberly Barr, LCGC