Individuals with Hb SS or Hb Sβ0 thalassemia are classified as having sickle cell anemia (SCA) because their hematological phenotype is characterized by a more severe chronic hemolytic anemia than those with Hb SC or Hb Sβ+ thalassemia, who have less hemolysis and less severe or no anemia. This evidence concerns the gene GSTM1 and sickle cell disease.