Klinefelter's syndrome, 47, XXY or XXY syndrome is a condition caused by a chromosome aneuploidy (change in the normal number of chromosomes). Affected males have an extra X sex chromosome. The principal effect is small testes development 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. It is the second most common extra chromosome condition, and is named after Dr. Harry Klinefelter, an endocrinologist at Massachusetts General Hospital who first described it in 1942. The condition exists in roughly 1 out of every 500 to 1,000 males.