GSTM1 and Schnyder corneal dystrophy: UCD in SCD has been attributed to polymerization of Hb S in the hyperosmolar, acidic and relatively hypoxic renal medulla, resulting in sludging of blood flow in the vasa recta, loss of medullary osmolar gradient, and eventual destruction of the vasa recta from vaso-occlusions/thrombosis [556].