(a) Diagram of the MSCVβ-globin/HS2/GFP vector. Arrows indicate transcription initiation sites. MSCV LTR, murine stem cell virus long terminal repeat; ψ+, extended packaging signal; βp, β-globin promoter; HS2, hypersensitive site 2; PGK, phosphoglycerate kinase promoter; HBPRE, the hepatitis B virus posttranscriptional regulatory element; PPT, polypurine track. (b) Flow cytometric profile of bone marrow cells 48 h postinfection with MSCVβ-globin/HS2/EGFP virus supernatant. The sorting gate used to select EGFP-positive cells is shown.