GSTM1 and thalassemia: Hb H disease is the most severe form of α-thalassemia compatible with post-natal life and is caused by the presence of α0-thalassemia deletion (or α-thalassemia-1) on one chromosome 16 and α+-thalassemia deletion (or α-thalassemia-2 i.e. -3.7 kb deletion and -4.2 kb deletion) on the homologous chromosome 16.