A sex chromosome anomaly caused by the presence of two extra X chromosomes in females (48,XXXX instead of 46,XX). This disorder is associated with delayed speech, learning difficulties, developmental delay and facial dysmorphism. Although disease severity is variable, the learning difficulties and developmental delay are generally mild to moderate. Commonly associated facial features include hypertelorism, upslanting palpebral fissures, epicanthal folds and a flat nasal bridge. Other anomalies may include dental abnormalities, hypotonia and joint laxity, radioulnar synostosis, heart defects, hip dysplasia, and ovarian dysfunction. An increased susceptibility to infections during childhood has also been reported.