Klinefelter syndrome is a genetic condition that results when a male is born with an extra copy of the X chromosome, resulting in an XXY configuration of the sex chromosomes, whereas normal males have an X and a Y sex chromosome (XY). Klinefelter syndrome can be caused by: One extra copy of the X chromosome in each cell (the most common cause), an extra X chromosome in some of the cells (mosaic Klinefelter syndrome, with fewer symptoms), and more than one extra copy of the X chromosome, which is rare and results in a severe form of the condition. Extra copies of genes on the X chromosome can interfere with male sexual development and fertility.
