The C-T substitution at position −158 of the Gy globin gene, referred to as the Xmn1-γ polymorphism, is a common sequence variant in all population groups, present at a frequency of 0.32 to 0.35.1 Clinical studies have shown that under conditions of hematopoietic stress, for example in homozygous β-thalassemia and sickle cell disease, the presence of the Xmn1-Gγ site favors a higher Hb F response. Here, GSTM1 is linked to sickle cell disease.