A rare chromosomal aberration characterized by the presence of three X (female) chromosomes. The condition does not exhibit a distinctive phenotype and majority of the affected females are physically and mentally normal. Abnormalities are infrequent and do not occur with any regularity–they include hypertelorism, epicanthal folds, depressed nasal bridge, kyphosis and variable other anomalies. Delay in growth and mental development, when present, is usually mild. Occasionally associated with Prader-Willi syndrome.