SYNDROME, KLINEFELTER

The most common single cause of hypogonadism (underfunction of the gonads) and infertility in men, Klinefelter syndrome is due to a chromosome abnormality with XXY (plus additional X or Y chromosomes). It affects about 1 in 500 males and results in small testes (hypogenitalism), underproduction of testosterone and infertility (hypogonadism), and a long-limbed, long-trunked, relatively tall, slim build. Klinefelter boys tend to have learning and/or behavioral problems. At adolescence there is little growth of facial hair and a third of boys develop gynecomastia (enlargement of the male breast). Named for the physician Harry Klinefelter who with E.C. Reifenstein, Jr. and Fuller Albright (the founder of modern endocrinology) described the condition in 1942 long before its chromosomal basis became known.