A 46,XX karyotype in phenotypic males occurs in one in twenty to twenty-four thousand male births. Such men have an absence of all female urogenital structures and have male psychosexual identification. The findings and hormonal profile resemble those in the Klinefelter's syndrome: the testes are small and firm (generally less than 2cm in length), gynaecomastia is common, the penis is normal, and azoospermia and hyalinisation of the seminiferous tubules are usual. Such men differ from Klinefelter men in that average height is less than normal, the incidence of mental deficiency is not increased, and hypospadias is common. Many XX men are positive for the SRY gene that is normally found on the Y-chromosome and thought to be responsible for testis differentiation. Thus, interchange of a fragment of a Y-chromosome with another chromosome may be a common cause.
