Klinefelter syndrome is characterized by an additional X chromosome, which becomes inactivated as a Barr body. Patients therefore have a 47, XXY karyotype and are phenotypically male. Klinefelter patients have abnormal Sertoli and Leydig cells, with accompanying small testes. Patients therefore do not produce as much testosterone and inhibin B, which leads to a number of laboratory findings. There is a loss of feedback inhibition, with increased FSH, LH, and estrogen levels . Klinefelter patients are also typically infertile, which is unsurprising given the degenerative seminiferous tubules and azoospermia. Pathology may reveal pseudoadenomatous Leydig cells and hyalinized seminiferous tubles / Sertoli cells, each of which is suggestive of Klinefelter syndrome. Patients typically have a characteristic physical appearance, notably tall stature and gynecomastia.