Definition of Klinefelter syndrome:

Klinefelter syndrome is the presence of an extra X chromosome in a male.

Causes, incidence, and risk factors:

Humans have 46 chromosomes. Chromosomes contain all of your genes and DNA, the building blocks of the body. Two of these chromosomes, the sex chromosomes, determine if you become a boy or a girl. Females normally have two of the same sex chromosomes, written as XX. Males normally have an X and a Y chromosome (written as XY).

Klinefelter syndrome results in males who have at least one extra X chromosome. Usually, this occurs as a result of one extra X (written as XXY).

Klinefelter syndrome is found in about 1 out of every 500-1,000 newborn males. Women who have pregnancies after age 35 are slightly more likely to have a boy with this syndrome than younger women.