relatively common (one per 500 live male births) human sex-chromosome (sex chromosome) disorder. It is characterized by the following: small testes, lack of sperm formation, late puberty with reduced secondary sexual characteristics (but with normal-sized penis), usually a delicate and long-limbed physique, occasionally breast enlargement (gynecomastia), and about a 25 percent frequency of mental retardation but a much higher frequency of behavioral difficulties.

Treatment with male hormone will normalize male appearance and produce a natural sex drive, but the patient will remain sterile.

The majority of Klinefelter individuals have one extra female sex chromosome, resulting in an XXY pattern and a total of 47 chromosomes instead of 46. Other, less frequent, chromosomal patterns include XXXY, XXXXY, XXYY, XXXYY, mixtures (mosaics), and partial losses or deletions. About 40 percent of the diagnosed examples of Klinefelter's syndrome have a normal XY pattern. One variant of Klinefelter's syndrome, the XXXXY type, is characterized by fusion of the forearm bones and other skeletal anomalies, underdevelopment of the penis and scrotum, incomplete descent of the testes, and marked mental retardation.

Diabetes mellitus and goitre incidence are more common, and various cancers may be more prevalent among Klinefelter's syndrome patients. Thyroidal trapping of radioactive iodine and the responses of the thyroid to injections of thyrotropin (TSH; thyroid-stimulating hormone) may be low. Testosterone production tends to be below normal, with urinary 17-ketosteroids in the normal or low-normal range. The serum and urinary gonadotropins follicle-stimulating hormone (FSH) and luteinizing hormone (LH) are usually, but not necessarily, increased.