The blood cells in individuals with hereditary spherocytic hemolytic anemia (HSHA) are sphere-shaped due to a defect within the blood cell as a result of a metabolic defect. Because of the cells’ shape, they are not readily passed through the small blood vessels of the spleen and are often prematurely destroyed by the spleen. The incidence rate of HSHA in the United States is estimated to be approximately 1 in 5,000 individuals and usually affects Caucasian individuals of northern European ancestry; it is rarely found in other racial groups. The disorder is autosomal dominant, with 50% of the siblings affected. However, in a small number of cases, neither parent has the defect and the expression of the disorder may be due to a recessive form of the disorder or to spontaneous mutation. The severity of the disorder depends upon whether the individual can compensate for the loss of red blood cells by producing more cells. If the bone marrow temporarily halts production of new blood cells due to infection, the individual may experience an aplastic crisis as a result of the loss of blood.