GSTM1 and thalassemia: In addition, each globin gene causative mutation is assigned to a single haemoglobinopathy group: (i) Thalassaemia, (ii) Structural Hb (including disease-causing variants, such as HbS and HbE) (iii) Thalassaemia and Structural Hb or (iv) HPFH and, subsequently, it is assigned to one or more haemoglobinopathy subgroups.