Fig. 4

a Viral titer (IU infectious units/ml) in conditioned medium from infected, but non-clonal populations of cultured packaging cells over time. Viral-producing cells that contained either the stably incorporated wild type retroviral construct (expressing ZsGreen, population A) or the hybrid-SIN vector (expressing both ZsGreen and turboRFP, population C) were obtained 21 days after transfection and cultured for an additional 35 and 63 days (56 and 84 days after transfection). Conditioned media was collected at the indicated time points and the viral titer compared to the titer at 21 days. A cells that integrated wild type retrovirus by infection; C cells that integrated the SIN retrovirus-piggyBac hybrid by transposition. b Viral titer (IU infectious units/ml) in conditioned medium from six randomly generated clones of packaging cells 84 days after transfection with plasmid C. Viral-producing cells that stably incorporated the minimal piggyBac/SIN retroviral construct (as determined by cells that expressed both ZsGreen and turboRFP (population C) 21 days after transfection) were purified and propagated in culture for an additional 63 days. The viral titer in the conditioned media from each clone at this time was determined as previously described for the non-clonal population of cells. (Data are expressed as mean ± SE; n = 4 experiments)