GSTM1 and thalassemia: HbE/β–thalassemia responders (n = 78) to HU showed the strongest association (p < 0.0001) between the Xmn1 polymorphism and an increase in HbF and Hb levels: a higher prevalence of Xmn1 was observed among responders, including 23.08% (n = 18) who were homozygous and 66.67% (n = 52) who were heterozygous for the Xmn1 polymorphism compared to 18% prevalence in the normal population (n = 100) and 15.62% in non-responders (n = 5).