Which form of sickle cell disease is the HbS mutation associated with?

The HbS (c.20A>T) mutation is the most severe mutation associated with sickle cell disease. If two copies of HbS are inherited, the most severe form of the disease (sickle cell anemia) will occur. If only one copy of HbS and one copy of a different mutation (HbC or HbE) is inherited, the disease symptoms are less severe.