Timeline for in vitro gene transfer and propagation of CAR⁺ T cells. The electrotransfer of transposon/transposase systems has narrowed the gap between nonviral and viral-based gene transfer for the amount of time in tissue culture needed to generate a clinically sufficient number of genetically modified CAR⁺ T cells. Cells transduced with virus (blue text) are typically propagated for 3 weeks before infusion.^68,69  T cells that undergo nonviral gene transfer with the SB system (red text) can be typically harvested within 4 weeks.