GSTM1 and Schnyder corneal dystrophy: The underlying pathophysiology of SCD stems from a single A > T point mutation in exon 1 of the HBB gene, which results in the sickle allele βs (NM_000518.4 (HBB):c.20 A > T), and the formation of the βs-globin chain that is incorporated in sickle Hb (HbS)2.