HBB and sickle cell disease: Sickle cell disease (SCD) refers to a group of inherited red blood cell disorders that occur due to mutations in β-globin genes, one of which is invariably haemoglobin S (HbS), a variant produced as a result of glutamic acid to valine substitution at position 6 of the beta-globin chain, the underlying mutation being HBB: c.20A > T.