Schematic maps of constructs used in the construction of packaging and producer cell lines. gagpol, rev, rTA, and VSV-G are γ-retroviral vector genomes, shown in their integrated, proviral form; ΔU3 denotes a SIN LTR, U3 denotes a non-SIN LTR, 7tetO indicates a doxycycline repressible promoter, and pac (puromycin N-acetyltransferase) confers resistance to puromycin. TL20 and PGK-ble are transfected DNA expression cassettes for a lentiviral vector genome and bleomycin resistance, respectively. The lentiviral vector genome produced from the doxycycline repressible TL20 cassette is identical to that produced by the CL20 vector plasmid used in transient transfections.