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

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.

The most common symptom is infertility. Other symptoms may include:

• Small, firm testicles
• Small penis
• Only a little bit of pubic, armpit, and facial hair
• Sexual problems
• Enlarged breasts ( gynecomastia)
• Tall height
• Abnormal body proportions (long legs, short trunk)

Adults may come to the doctor because of infertility. School-age children may be brought in to because of learning problems.

The following test results may be found:

• Karyotyping shows 47 XXY.
• Semen count is low.
• Serum testosterone is low.
• Serum luteinizing hormone is high.
• Serum follicle stimulating hormone is high.
• Serum estradiol levels (a type of estrogen) is high.