GSTM1 and Schnyder corneal dystrophy: SCD is caused by a single mutation in the β-globin gene that changes the sixth amino acid in the β-globin protein of hemoglobin from glutamic acid to valine (31), which makes RBCs highly susceptible to sickling due to the production of sickle hemoglobin (Hb S) and thus impairs Hb capacity to deliver oxygen to tissues.