GSTM1 and thalassemia: Seventy-two percent (1633 participants) had a diagnosis of sickle cell anemia (SCA) which includes the 2 most severe genotypes of SCD (1545 patients [68.3%] with hemoglobin [Hb] SS, the most severe form of SCD, and 88 patients [3.9%] with HbSβ0 thalassemia).