GSTM1 and alpha thalassemia spectrum: The severity of α-thalassemia can be categorized into four groups: the silent carrier (–α/αα), also known as α+-thalassemia; the α-thalassemia trait, including homozygous α+-thalassemia (–α/–α) and heterozygous α0-thalassemia (– –/αα); hemoglobin (Hb) H disease (– –/–α); and homozygous α0-thalassemia (Hb Bart’s disease) (– –/– –).