Triple X syndrome is an abnormality of the chromosomes that affects about 1 in 1,000 females. In triple X syndrome, a female has three X chromosomes ? hence, the name. Triple-X syndrome is listed as a "rare disease" by the Office of Rare Diseases (ORD) of the National Institutes of Health (NIH). Approximately 0.10% or 1 per 1,000 newborn girls suffer from triple X syndrome.