Alpha thalassemia is a genetic disorder of hemoglobin production that typically is inherited in an autosomal co-dominant fashion. Rare forms of alpha-thalassemia, however, occur as de novo or acquired disorders. These disorders occur in two clinical situations: 1) alpha-thalassemia associated with mental retardation, and 2) acquired alpha-thalassemia (HbH disease) associated with myelodysplastic syndrome. Study of these rare disorders has led to the identification and characterization of a gene on the X chromosome (called ATRX) that encodes a trans-acting factor capable of influencing the expression of alpha-globin and other genes.