In the United States, most individuals with SCD have homozygous hemoglobin (Hb) SS disease (Hb SS), and the remainder have a compound heterozygous form of SCD caused by co-inheritance of Hb S with a different beta globin mutation such as Hb C or a variety of β thalassemia mutations4. The gene discussed is GSTM1; the disease is Schnyder corneal dystrophy.