HBB and Schnyder corneal dystrophy: However, SCD can also result from the inheritance of βS in combination with a wide range of other HBB mutations, the two most common being a second structural β-globin variant βC (SCD SC)3 and one of the many β-thalassaemia mutations that lead to the reduced production of normal β-globin (SCD S/β-thalassaemia).4 SCD SS is the most severe form of SCD and, consequently, is the main focus of the current review.