XXY syndrome

n. A genetic disorder of males characterized by the presence of an extra X, or female sex, chromosome in all the cells of the body. Such males usually have very small testes, do not produce sperm, have enlarged breasts, and produce follicle stimulating hormone. Behavioral problems are common among boys with this disorder, and it is not known whether they are due to the genetic abnormality or the difficulty in social adjustment caused by being physically different from other boys. Also called Klinefelter syndrome.