Klinefelter Syndrome is a common chromosomal variation. It is also known as 47,XXY, XXY Syndrome or XXY. On this site we use the terms interchangeably.
In most individuals there are 46 chromosomes, arranged as XY or XX. When a person is born with an extra sex chromosome (known as the X or Y chromosome) it is called a sex chromosome variation or anomaly. A person with Klinefelter Syndrome (KS) has an extra X chromosome to make up the genetic profile 47, XXY. The majority of individuals will have the extra X chromosome in all their cells but 15% to 20% of KS individuals are mosaics usually with 2 cell lines 47, XXY/46, XY. This essentially means that the extra X is in some of their body cells but not all of them.

Klinefelter syndrome (KS) was first described by Harry F. Klinefelter in 1942. He reported 9 men with testicular abnormalities who failed to produce ***** and had gynecomastia. In 1959, this was found to be the result of an additional X chromosome.