Fig. 2

a Percentage of ZsGreen, turboRFP or ZsGreen/turboRFP positive packaging cells 48 h after transfection (i.e. transiently positive) with tested plasmids. RV wild type retroviral vector A, RV(SIN) SIN retroviral vector B, RV(SIN)minPB SIN retroviral/minimal piggyBac hybrid vector C, Control negative control vector D. b, c The viral titers (IU infectious units/ml) in the medium of two sequential passages of retrovirus originated from transiently transfected plasmids. Viral conditioned medium was collected from virus-producing cells 48 h after transfection with plasmids A–D and then applied to other packaging cells (1st infection cycle, b). Only cells expressing the viral marker (ZsGreen) after the 1st infection cycle were purified and their conditioned medium tested for retroviral SIN function using new target cells (2nd infection cycle, c). (Data are expressed as mean ± SE; n = 4 experiments; *P < 0.05)