Klinefelter syndrome is a chromosomal condition that affects male sexual development. Most males with Klinefelter syndrome have one extra copy of the X chromosome in each cell. Because their testicles do not develop normally, affected males may have low levels of the hormone testosterone which can lead to breast development, reduced facial and body hair, and infertility.
Most often, Klinefelter syndrome is caused by a single extra copy of the X chromosome in all cells of the body (47, XXY). Occasionally, individuals will have the extra copy in only some of their cells (mosaic 46, XY/47,XXY) and other KS individuals will have several extra copies of the X chromosome or extra copies of both the X and Y chromosomes in all of the body's cells. Symptoms of KS can vary depending on the number of extra copies of the X chromosome. This condition is not inherited, but usually occurs as a random event during the formation of reproductive cells (eggs and sperm). An error in cell division called non-disjunction can result in reproductive cells with an abnormal number of chromosomes.