Klinefelter's syndrome is a relatively common genetic disease 
affecting about 1 out of 500 males. Typically an affected male 
will look and behave normally from infancy through childhood. 
Diagnosis is rarely made before puberty, at which time symptoms 
such as small size and firmness of testes, lack of sperm 
production, little or no body hair, and breast development become 
apparent. Mental retardation often occurs. Men with this 
disorder are likely to be very tall and may develop obesity, 
osteoporosis, or psychosis. 

The disease is caused by an extra X, or female, sex chromosome 
in the body cells. Normally, a male has one X chromosome and one 
Y chromosome, the latter determining maleness. Body cells in 
patients with Klinefelter's syndrome have one or more extra X 
chromosomes along with the Y. The extra X chromosomes interfere 
with the development of male characteristics. Generally, as the 
number of X chromosomes increases, the severity of mental 
retardation and malformation also increases. Treatment involves 
administering male hormones, which produce more masculine physical 
traits, although sterility can never be reversed.