Different clinical presentation of Klinefelter's syndrome in monozygotic twins

There is a wide variability in the clinical presentation of Klinefelter's syndrome. We report the case of a 45-year-old man who was incidentally diagnosed a 47,XXY/46,XY karyotype in a bone marrow aspiration (case 1). He presented hypogonadic features with undetectable testosterone levels and a height in accordance with mid-parental height. He had a monozygous sibling (case 2) who did not show clinical signs of hypogonadism and whose height exceeded mid-parental height. Both patients had presented language disorders since childhood. The karyotype of lymphocytes in peripheral blood of both subjects was compatible with mosaic Klinefelter's syndrome (46,XY/47,XXY). Testosterone replacement was initiated in case 1. Lack of testicular involvement due to mosaicism and the overexpression of the SHOX gene in case 2 could explain the marked differences in phenotype in these homozygous twins.