GSTM1 and thalassemia: The most severe clinical phenotype of β-thalassemia, β-thalassemia major, can result from any one of several causes, including the complete absence of β-globin chains (β0-thalassemia) and co-inheritance of a β-thalassemia mutation and hemoglobin (Hb) E, a common β-globin variant in Asians, (Hb E/β-thalassemia).