Insertion of a normally functioning human β-globin gene into the hematopoietic stem cells (HSC) of patients with β-thalassemia may be an effective approach to the therapy of this disorder. Safe, efficient gene transfer and long-term, high-level expression of the transferred human β-globin gene in animal models are prerequisites for HSC somatic gene therapy. We have recently shown for the first time that, using a modified β-globin retroviral vector in a mouse transplant model, longterm, high-level expression of a transferred human β-globin gene is possible. The human β-globin gene continues to be detected up to eight months post-transplantation of β-globin-transduced hematopoietic cells into lethally irradiated mice. The transferred human β-globin gene is detected in three of five mice surviving long-term (>4 months) transplanted with bone marrow cells transduced with high-titer virus. The unrearranged 5.1 kb human β-globin gene-containing provirus is seen by Southern blotting in two of these mice. More importantly, long-term expression of the transferred gene is seen in two mice at levels of 5% and 20% that of endogenous murine β-globin. We document stem cell transduction by showing continued highlevel expression of the human β-globin gene in secondarily transplanted recipient mice. These results provide evidence of HSC transduction with a human β-globin gene in animals and demonstrate that retroviral-mediated unrearranged human β-globin gene transfer leads to a high level of human β-globin gene expression in the long term for the first time. A gene therapy strategy may be a feasible therapeutic approach to the β-thalassemias if consistent human β-globin gene transfer and expression into HSC can be achieved. P atients with β-thalassemia can be cured by allogeneic stem cell transplantation. 1,2 However, this treatment modality is associated with significant morbidity and mortality and is limited by the number of available HLA-compatible siblings. 3 Another potential approach to curing β-thalassemia is by the insertion of a normally functioning human β-globin gene into a 180 ANNALS NEW YORK ACADEMY OF SCIENCES FIGURE 1. A diagram of the p141 vector. The vector is flanked by the 5′ long terminal repeat (LTR) and 3′ deleted long terminal repeat (∆ LTR). The extended packaging signal is represented by ψ+. The β-globin gene is in reverse orientation with exons ( ), introns ( ), the deleted 3′ enhancer ( ) and the 372 bp deletion in intron 2 (∆ Rsa I). The LCR core sequences HS234 ( ) and the neomycin resistance gene (NEO) with the PGK promoter ( ) are as indicated.