Schematic diagram of α- and β-globin gene clusters and the types of hemoglobin produced at each developmental stage. Genes are arranged along the chromosome in the order in which they are expressed during development; (A) in the α-cluster ζ (embryonic) and α (fetal and adult); (B) in the β-cluster ε (embryonic), γ (fetal), and δ and β (adult). The 4 upstream regulatory elements of the α-locus are known as MCSR1 to MCSR4, whereas the 5 regulatory elements of the β-locus are collectively referred to as locus control region (LCR). The hemoglobin types expressed during different stages of development are embryonic (Hb Gower-I [ζ₂ε₂], Hb Gower-II [α₂ε₂], and Hb Portland [ζ₂γ₂]), fetal (HbF [α₂γ₂]), and adult (HbA [α₂β₂] and HbA₂[α₂δ₂]).