Klinefelter syndrome has a prevalence of approximately 1:1,000 in male newborns. 80% of affected individuals have a pure 47,XXY karyotype, while the remaining 20% have mosaicism or other X polysomies. In childhood, discrete abnormalities in psychomotor development may be present, as well as passive behavior and learning difficulties. During puberty, hypergonadotropic hypogonadism develops with the underdevelopment of secondary sexual characteristics and azoospermia, as well as tall stature with increased fat in the trunk and possibly gynecomastia. Osteoporosis may occur later. Intelligence is usually within the normal range or 5-10 IQ points below the family average. From puberty onward, testosterone substitution should be performed where there are subnormal values and for osteoporosis prophylaxis.
Ogata et al. 2001, Am J Med Genet 98:353 / Ogata et al. 2001, J Med Genet 38:1