Triple X syndrome (also known as triplo-X, trisomy X, XXX syndrome, 47,XXX aneuploidy) is a form of chromosomal variation characterized by the presence of an extra X chromosome in each cell of a human female. A mosaic form also occurs where only a percentage of the body cells contain XXX while the remainder carry XX. The extent to which an individual is affected by the condition will depend upon the proportion of XXX to XX throughout. Triple X results during division of a parent's reproductive cells and occurs about once in every 1,000 female births. Unlike most other chromosomal conditions (such as Down syndrome), there is usually no distinguishable difference to the naked eye between those with triple X and the rest of the female population.