A syndrome in which the affected patients have only 45 chromosomes, the loss of one of the X chromosomes producing an XO chromosome constitution. The syndrome occurs in 1/2500 female births and nearly 99% of XO fetuses are aborted. Gonadal agenesis and short stature are the main features in the surviving infants. Associated anomalies may included webbed neck, cubitus valgus, shield chest, short stature, lymphedema, coarctation of the aorta, pigmented nevi, and various renal, skeletal, dermatologic, neoplastic, and autoimmune complications. Mental retardation is attributed to ring chromosome X. The phenotype varies and not all abnormalities occur in all patients.