What Is Klinefelter Syndrome?

Klinefelter syndrome (KS) is a genetic condition in which boys are born with an extra copy of the X chromosome (piece of genetic material). A person with KS has 47 chromosomes inside their cells instead of the usual 46. KS is also called 47, XXY.

Males with KS typically have small testicles that make less testosterone (male sex hormone) than usual. Many men also have low sperm counts that can lead to infertility.

Other common challenges in males with KS include learning disabilities and trouble with social interactions.

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Rev. 1/2020. MassGeneral Hospital for Children and Massachusetts General Hospital do not endorse any of the brands listed on this handout. This handout is intended to provide health information so that you can be better informed. It is not a substitute for medical advice and should not be used to treatment of any medical conditions.