Klinefelter’s syndrome occurs due to an atypical sex chromosome pattern which affects the development of males who have an extra X chromosome (XXY). Klinefelter’s syndrome is found in around 1 in 1,000 males. The most common symptom is infertility, but males with Klinefelter’s syndrome may also have small, firm testes, a small penis, sparse pubic, armpit and facial hair, enlarged breasts (called gynecomastia), tall stature, and abnormal body proportions (long legs, short trunk).