Cooley's anemia (beta-thalassemia major, hereafter referred to as thalassemia) is a severe, inherited blood disorder characterized by a quantitative defect in the synthesis of the beta chain of hemoglobin caused by any 1 of more than 100 known mutations in and around the beta globin gene cluster. The disease is characterized by severe anemia beginning in the first 6 to 12 months of life. If untreated, the life expectancy is less than 5 years of age. Chronic red blood cell transfusions to maintain hemoglobin levels between 9 and 11 gm/dl ("hypertransfusion") alleviate the anemia and partially suppress erythropoiesis. The regular administration of red blood cells also improves growth, delays or prevents enlargement of the liver and spleen, and prevents the development of bone abnormalities that cause fractures as well as disfiguring changes known as Cooley's facies. Transfusions carry risks of alloimmunization, iron overload, and blood-transmitted infections. In the absence of effective iron chelation therapy, iron overload leads to numerous complications, including delayed or absent sexual development, diabetes mellitus, cirrhosis, cardiac arrhythmias, and congestive heart failure. Nonchelated or poorly chelated patients usually die of heart disease by 20 to 30 years of age.