Vector schematic: general organization of proviral form of lentiviral vectors for gene therapy for SCD. (A) Antisickling β-globin vector containing the T87Q mutation. The SIN lentiviral vector is noninsulated and contains a modified LCR with HS2, HS3, and HS4. (B) Antisickling β-globin vector containing the G16D, E22A, and T87Q mutations (AS3). The SIN lentiviral vector has an FB element for enhancer-blocking activity and contains a modified LCR with HS2, HS3, and HS4. (C) A γ/β-globin hybrid vector containing the coding regions of γ-globin and the noncoding regions of β-globin. γ-globin (dark blue boxes), β-globin, or antisickling β-globin gene cassettes (blue boxes) with β-globin 5′ and 3′ untranslated regions (short blue boxes) under the control of a β-globin promoter (blue arrow) and modified β-globin LCR (red). Gene cassette is in reverse with respect to viral transcription to avoid aberrant splicing during packaging due to presence of globin intronic sequences (light blue boxes). All vectors are SIN (ΔU3) lentiviral vectors (green). Vectors are intended as basic schematics of gene therapy clinical vectors for SCD and are neither drawn to scale nor are all details included. ψ, packaging signal; cPPT, central polypurine tract; HS, DNase I hypersensitive site; LTR, long terminal repeats (U3, R, and U5); RRE, rev-responsive element; WPRE, woodchuck hepatitis virus posttranscriptional regulator element.