A sex chromosome aneuploidy with an incidence of 1 in 85,000 male births. Principal findings include radioulnar synostosis, hypogonadism, mental retardation, and speech disorders. Associated disorders ara variable and may include characteristic facies, multiple skeletal abnormalities, and cardiac and genital malformations. Many features are similar to those seen in the Klinefelter syndrome (chromosome XXY syndrome), hence the synonym Klinefelter variant.