Genetics Home > Klinefelter Syndrome

Klinefelter syndrome is a genetic condition affecting male sexual development (females are not affected). Typical symptoms of this condition include breast development, small testes, and sparse facial and body hair. Treatment options include testosterone, surgery, and counseling. Although men with this condition are at risk for other medical problems, many men who have it lead full and active lives, and can expect a normal lifespan.

What Is Klinefelter Syndrome?

Klinefelter syndrome is a chromosomal condition that affects male sexual development. Most males with this condition have an extra X chromosome in each cell. Because their testicles do not develop normally, affected males may have low levels of the hormone testosterone when they reach puberty.

How Common Is It?

Klinefelter syndrome affects 1 in 500 to 1,000 males. Variants of this condition are much rarer, occurring in 1 in 50,000 or fewer male births. Females are not affected by this syndrome.

What Causes It?

The cause of Klinefelter syndrome is the presence of one (or more) extra X chromosome in a male's cells. Extra copies of genes on the X chromosome interfere with male sexual development, preventing the testicles from functioning normally and reducing the levels of testosterone in the body.
While the condition is genetic (in that it involves an extra X chromosome gene), it is not usually hereditary in the conventional sense. That is, the syndrome is not inherited, nor does it "run in families." Klinefelter syndrome is usually the result of a random event that occurs during the formation of reproductive cells (eggs and sperm).
(Click Genetic Cause of Klinefelter Syndrome for more information.)
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Last reviewed by: Arthur Schoenstadt, MD
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