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.