 4. Acceptance of hydroxyurea therapy for at least three years. At 36 months, a decision between the family and provider can be made whether to continue with hydroxyurea therapy until the end of the study 7312020. If the study is terminated early, families will be informed of optimal dose and given the option to continue hydroxyurea therapy until 7312020. The provisions of hydroxyurea will be continued to assess the toxicity of the study medication among study groups. 