The three most common thalassemia genotypes found in Kantharalak, Nam Khun, Nam Yuen, and Na Chaluai were heterozygous α+-thalassemia (αα/− α3.7, βA/βA), double heterozygous for α+-thalassemia and Hb E (αα/− α3.7, βA/βE), and heterozygous Hb E (αα/αα, βA/βE). This evidence concerns the gene GSTM1 and thalassemia.