The human α-globin gene cluster on chromosome 16 and the β-globin gene cluster on chromosome 11. The α-globin gene cluster contains three functional globin genes, the embryonic ζ gene (HBZ) and two fetal/adult α, α1 and α2, genes (HBA1 and HBA2) (A). The β-globin gene cluster contains five functional genes, the embryonic ε gene (HBE), two fetal Gγ and Aγ genes (HBG2 and HBG1), and adult δ and β (HBD and HBB) genes (B). HS-40 and the locus control region (LCR) regulate α- and β-globin gene expression, respectively. Hemoglobin differentially expressed at embryonic, fetal, and adult stages are represented (C).