Klinefelter's syndrome is a genetic disorder which means that men have an XXY chromosome set up rather than the usual XY (for a male). The XXY almost always causes feminisation of men or anti-masculinisation. Klinefelter's syndrome can be extreme or non-apparent depending on the person. Usual symptoms can be :
Voice not breaking
Long arms or legs
testicular atrophy (small testicles)
Crossdresser's would probably like to have klinefelter's syndrome so that they would look girlier.