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Klinefelter syndrome

Other Names for this Disease
  • Klinefelter's syndrome
  • XXY syndrome
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What causes Klinefelter syndrome?

Klinefelter syndrome is not inherited; it usually occurs as a random event during the formation of reproductive cells (eggs and sperm). An error in cell division called nondisjunction results in a reproductive cell with an abnormal number of chromosomes. For example, an egg or sperm cell may gain one or more extra copies of the X chromosome as a result of nondisjunction. If one of these atypical reproductive cells contributes to the genetic makeup of a child, the child will have one or more extra X chromosomes in each of the body's cells.[1]

Most often, Klinefelter syndrome is caused by a single extra copy of the X chromosome, resulting in a total of 47 chromosomes per cell. Males normally have one X chromosome and one Y chromosome in each cell (46, XY), while females have two X chromsomes (46, XX). Individuals with Klinefelter syndrome usually have two X chromosomes and one Y chromosome (47, XXY). Some individuals with Klinefelter syndrome have the extra X chromosome in only some of their cells; these individuals are said to have mosaic Klinefelter syndrome.[1]

It is estimated that about half of the time the cell division error occurs during development of the sperm, while the remainder are due to errors in egg development. Women who have pregnancies after age 35 have a slightly increased chance of having offspring with this syndrome.[2]

The features of Klinefelter syndrome are due to the extra copies of genes on the extra X chromosome, which can alter male sexual development.[3]
Last updated: 9/18/2013

  1. Klinefelter syndrome. Genetics Home Reference (GHR). July 2008; Accessed 12/10/2008.
  2. Learning about Klinefelter syndrome. National Human Genome Research Institute (NHGRI) Web site. May 27, 2008; Accessed 12/10/2008.
  3. Kirmse B. Klinefelter syndrome. MedlinePlus Web site. December 11, 2006; Accessed 4/1/2008.