The term SCD encompasses sickle cell anemia (SCA) (Hb SS), hemoglobin S combined with hemoglobin C (Hb SC), hemoglobin S associated with β-thalassemia (Sβ0 Thal and Sβ+ Thal), and other double heterozygous conditions that cause clinical disease. This evidence concerns the gene GSTM1 and autosomal dominant cerebellar ataxia.