Aidan watched as his friends all gradually got deeper voices and
body hair. He kept waiting for his turn, but his body didn't
really change much. Aidan's doctor took a sample of his blood
for testing. The test showed that Aidan had a condition called
Klinefelter syndrome. What Is It? Klinefelter syndrome can cause
problems with learning and sexual development in guys. It's a
genetic condition (meaning a person is born with it).
Klinefelter syndrome only affects males. It happens because of a
difference deep inside the body's cells, in microscopic centers
called chromosomes. Chromosomes are tiny, but they play a huge
role in who we are — including deciding our gender, how we look,
and how we grow. Doctors sometimes call Klinefelter syndrome
"XXY." Instead of one X and one Y chromosome like most guys
have, guys with Klinefelter are born with an extra "X"
chromosome in most or all of their cells. When puberty starts
and guys' bodies begin to make sex hormones, boys with
Klinefelter usually don't produce as much