- Abstract -

Klinefelter's syndrome, also known as 47, XXY or XXY syndrome is a condition in which males have an extra X sex chromosome. It is the most common chromosomal disorder associated with male hypogonadism and infertility. In the same time, it is the second most common condition caused by the presence of extra chromosomes. The syndrome is characterized by hypogonadism (small testes, azoospermia/oligospermia), gynecomastia in late puberty, psychosocial problems, hyalinization and fibrosis of the seminiferous tubules, and elevated urinary gonadotropin levels.

The most common karyotype is 47,XXY, which accounts for 80-90% of all cases. Mosaicism (46,XY/47,XXY) is observed in about 10% of cases. Other variant karyotypes, including 48,XXYY; 48,XXXY; 49,XXXYY; and 49,XXXXY, are rare. The genetic variation is irreversible. Testosterone treatment is an option for some individuals who desire a more masculine appearance and identity.

Key words: Klinefelter syndrome, phenotype, child.