Klinefelter's synrome, 47,XXY or XXY syndrome is a condition caused by a chromosome aneuploidy. Affected males have an extra X sex chromosome. The principal effects are development of small testicles and reduced fertility. A variety of other physical and behavioral differences and problems are common, though severity varies and many boys and men with the condition have few detectable symptoms. The second most common extra chromosome condition, it is named after Dr. Harry Klinefelter, an endocrinologist at Massachusetts General Hospital, Boston, Massachusetts, who first described it in 1942. The condition exists in roughly 1 out of every 500 males. Because of the extra chromosome, individuals with the condition are usually referred to as "XXY Males", or "47, XXY Males".