48, XXXY syndrome is a sex chromosome aneuploidy in which males have two extra X chromosomes. Human cells usually contain two sex chromosomes, one from the mother and one from the father. Usually, females have two X chromosomes (XX) and males have one X and one Y chromosome (XY). The appearance of at least one Y chromosome with a properly functioning SRY gene makes a male. Therefore, XXXY only affects males. Males affected with XXXY syndrome have 48 chromosomes instead of the typical 46. This is why XXXY syndrome is sometimes written as 48, XXXY syndrome. It is estimated that XXXY affects one in every 50,000 male births.