Tetrasomy X is a chromosome disorder that only affects females and is caused by having four copies of the X chromosome instead of two. Females with Tetrasomy X have a total of 48 chromosomes in their cells, so this condition is sometimes written as 48, XXXX. The signs and symptoms of Tetrasomy X vary, but can include mild to moderate speech and learning difficulties; developmental delay; distinctive facial features; dental abnormalities; hypotonia and joint laxity; radioulnar synostosis; heart defects; hip dysplasia; and problems with ovarian function. An increased risk of childhood infections has also been reported. Tetrasomy X is caused by a random error that occurs during the development of an egg cell and is not caused by anything a mother does during her pregnancy.