Transgene constructs. All transgene constructs contain a transcription unit that comprises a truncated β-globin promoter (βpr; □) and γ-globin coding segment (γ gene) containing a partial deletion (▿) of the second intron (▪, exons; black line, untranslated regions). Construct HS-40/βpr/γ gene contains the HS-40 element from the human α-globin locus (αHS-40; ▦). Construct HS-40(mut)/βpr/γ gene contains the HS-40 element with a G-to-T transversion in the 3′ NF-E2 (nuclear factor-erythroid 2) motif. Construct μLCR/βpr/γ gene contains a 2.5-kb composite μLCR (). Construct cHS4/HS-40/βpr/γ gene contains HS-40 and a 1.2-kb fragment encompassing the cHS4 chromatin insulator (▨). Construct cHS4/HS-40/βpr/γ gene contains HS-40 and a 1.2-kb fragment encompassing the cHS4 chromatin insulator (▧).