A diagram of the p141 vector. The 5′ long terminal repeat (LTR) and 3′ deleted long terminal repeat (Δ LTR) are indicated. Ψ+ represents the extended packaging signal. The β-globin gene is shown in reverse orientation with exons (▧), introns (▨), the deleted 3′ enhancer (⊠), and the 372-bp deletion in intron 2 (Δ Rsa I). LCR elements HS234 (▪) and the neomycin resistance gene (NEO) with the PGK promoter (▨) are indicated.