HBB and Schnyder corneal dystrophy: Sickle cell disease (SCD) is caused by a single nucleotide substitution in the beta-globin gene (haemoglobin subunit beta; HBB) (Fig. 1A), which encodes a component of haemoglobin (Hb), the protein complex that constitutes 70% of red blood cells (RBCs) and is responsible for transporting oxygen to all organs of the body.