The term “Klinefelter syndrome,” or KS, describes a set of features that can occur in a male who is born with an extra X chromosome in his cells. It is named after Dr. Henry Klinefelter, who identified the condition in the 1940s.1
Usually, every cell in a male’s body, except sperm and red blood cells, contains 46 chromosomes. The 45th and 46th chromosomes—the X and Y chromosomes—are sometimes called sex chromosomes, because they determine a person’s sex. Normally, males have one X and one Y chromosome, making them XY. Males with KS have an extra X chromosome, making them XXY.
KS is sometimes called 47,XXY (“47” refers to total chromosomes) or the XXY condition. Those with KS are sometimes called XXY males.
Some males with KS may have both XY cells and XXY cells in their bodies. This is called “mosaic.” Mosaic males may have fewer symptoms of KS, depending on the number of XY cells they have in their bodies and where these cells are located. For example, males who have normal XY cells in their testes may be fertile.2
In very rare cases, males might have two or more extra X chromosomes in their cells (e.g., XXXY, XXXXY) or an extra Y (e.g., XXYY). This is called poly-X KS, and it causes more severe symptoms.1