There are 4 clinical definitions of α-thalassemia syndromes: 1) silent carrier, defined as heterozygous α+-thalassemia (-α/αα) with mostly normal hemoglobin or mild hypochromic anemia; 2) α-thalassemia trait, defined as heterozygous α0-thalassemia (--/αα) or homozygous α+-thalassemia (-α/-α) with mild anemia; 3) HbH disease, defined as compound heterozygous α+/α0-thalassemia with 3 inactive α-genes (--/-α) with moderate hemolytic anemia; and 4) Hb Bart’s, defined as homozygous α0-thalassemia (--/--) with hydrops fetalis. The gene discussed is GSTM1; the disease is hydrops fetalis.