Klinefelter Syndrome

The approximately 1 in 500 males with Klinefelter syndrome – also known as XXY males – have an extra X chromosome. For information about the effects of XXY or to find support resources, check these sites.
Klinefelter Syndrome and Associates
Information and support services from a nonprofit organization.
Klinefelter Syndrome & The XXYY Variant
A page by a parent of a child with XXYY, a very rare variant of Klinefelter syndrome.