1. Homozygous SS sickle cell children, aged > 3 years, of sub-Saharian Africa extraction, in a steady-state of disease (free of any infectious or vaso-occlusive events for the 4 weeks prior to and 2 weeks after blood sampling, and transfusion-free for 4 months prior to blood sampling), taken no drug except penicillin-V, folate or iron supplementation, hydroxyurea, divided into three groups :