(A) Prototypic design of a γ-retroviral vector β/γ globin gene and the small LCR core elements. The β-globin gene (exons depicted in blue shaded boxes) is placed in the reverse orientation in the retroviral vector to prevent splicing of the introns in the viral genomic RNA during production of the virus. Arrow indicates the direction of transcription. In the γ-retroviral vector construct, HS2, HS3, and HS4 denotes LCR core derivatives.¹²  (B) Successful β/γ-globin LV vectors used by different groups. TNS9 vector is based on the publication by May et al.²⁴  and encodes the human beta globin gene. The β^A87Q vector encodes a mutated β-globin gene at the 87th codon, conferring it with anti-sickling properties,⁵⁵  and also carries two copies of the 5′ 250-bp cHS4 insulator core. This vector was used for the gene therapy trial for β-thalassemia in France led by Dr. Philippe Leboulch. The BG-I vector, described by Putheenvetil et al.,³²  expresses β-globin from a 254-bp β-globin promoter, contains large LCR elements, and is flanked by the 1.2-kb full-length cHS4 insulator in the LTRs. The vector d432β-^Aγ vector²⁸  also contains γ-globin 3′ regulatory untranslated regions (shaded purple) and enhancer elements downstream of the γ-globin coding sequences (3′e*). mLARβΔγV5 vector³⁰  consists of extended regions of β-globin LCR. The GLOBE vector is described in the article by Miccio et al.²⁹  Triangles represent deletion of the 372-bp purine-rich sequences in the β-globin IVS 2 (a 562-bp deletion of IVS2 in the GLOBE vector) and the 400-bp enhancer deletion in the LTR to generate a self-inactivating vector design. RRE, rev response element; HS2, HS3, and HS4, DNase hypersensitive sites 2, 3, and 4 present in the β-globin LCR and the length of respective hypersensitive sites are indicated adjacent to them; cPPT, central polypurine tract; LTR, long terminal repeat; 3′e, β-globin 3′ enhancer element; ψ, packaging signal; Ψ⁺, extended packaging signal; βP, β-globin gene promoter. The β or γ globin genes are depicted in the reverse orientation, with β-globin exons in blue, γ-globin exons in green, and β-globin introns shown as unshaded boxes