This article waslast modified on July 10, 2017.

A rare genetic condition in boys and men caused by an extra X chromosome. (Males normally have one X and one Y chromosome.) The presence of the extra X chromosome may or may not produce obvious signs and symptoms (usually in teens and adults), such as low testosterone, small penis and testicles, enlarged breasts, tall stature and/or behavioral, learning, speech or language disabilities. Most of these individuals are infertile.