<qa>  What is the protein in red blood cells that carries oxygen to the body?Patients with sickle cell disease have abnormal hemoglobin the protein in red blood cells that carries oxygen to the body. This abnormality causes red blood cells to take on a sickle shape, producing disease symptoms. Fetal hemoglobin, a type of hemoglobin present in fetuses and babies, can prevent red cells from sickling. The drug hydroxyurea increases fetal hemoglobin production in patients with sickle cell disease by making a molecule called nitric oxide. The drugs L-arginine and Sildenafil Viagra increase the amount or the effect of nitric oxide. This study will evaluate: 