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                                        Inclusion Criteria
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1. Patients with sickle cell anemia. 2. History of hydroxyurea use with evidence of drug related myelosupression, such as an absolute neutrophil count (ANC) of less than 4000/UL. 3. Age between 3 years and 22 years inclusive. 4. Or patients with sickle cell anemia and a history of overt clinical stroke.                                      |