(A) Structure of the pSF-EGFP1 retroviral vector. EGFP, enhanced green fluorescent protein; LTR, long terminal repeat; Ψ⁺ indicates the presence of the retroviral packaging sequence; PCMV, PCC4-cell-passaged mutant virus; SFFVp, polycythemic strain of the spleen focus-forming virus. (B) Proviral form of the pSF-EGFP1 vector following retroviral integration. PBS, primer binding site; MESV, murine embrionic stem cell virus.