a) Schematic drawing of the lentiviral vectors used. LV indicates conventional integrating lentiviral vector and NILV indicates non-integrating lentiviral vector, generated by using packaging plasmid with deficient integrase. S/MAR indicates that the vector contains a scaffold matrix attachment region element and S/MARrev means that the element is positioned in the reverse orientation. LTR: Long terminal repeat; CMV prom: cytomegalovirus immediate early gene promoter; GFP: green fluorescent protein; 2A: viral self-cleaving T2A sequence; puro: puromycin. 