GSTM1 and thalassemia: The Hb Sβ thalassemias are divided into two groups depending on the severity of the β thalassemia mutation; mutations that result in absent production of β globin are termed Hb Sβ0 thalassemia while those that result in reduced, but not absent, production of β globin are termed Hb Sβ+ thalassemia.