Triple X syndrome: What causes it?

What causes XXX syndrome in girls?

- No name / No state given

Mayo Clinic pediatrician Jay Hoecker, M.D., and colleagues answer select questions from readers.

Answer

XXX syndrome, also called triple X syndrome, is a rare chromosomal abnormality that affects only females. As the name implies, females with XXX syndrome have three X chromosomes instead of the usual two. The cause of this gene defect isn't known.

XXX syndrome is the most common X chromosome disorder in females. In many cases, XXX syndrome causes no signs or symptoms. When signs and symptoms do appear, they may include:

Tall stature
Small head (microcephaly)
Vertical skinfolds that may cover the inner corners of the eyes (epicanthal folds)
Delayed development of certain motor skills, speech and language
Learning disabilities such as dyslexia
Infertility

A doctor may make a diagnosis of XXX syndrome by a chromosome analysis of a blood sample. The disorder may be found during prenatal genetic testing, such as amniocentesis. Often, XXX syndrome is identified in older children as part of an evaluation of developmental delay.

A female with XXX syndrome may be referred to a pediatric geneticist for counseling. There's no cure for XXX syndrome. When possible, treatment is directed at managing the signs and symptoms of the disorder.