On the day of transfection cells were tripsinized, counted, resuspended in 100 L of nucleofector solution (Basic Nucleofector Kit, Prim. Fibroblasts; Amaxa, Cologne, Germany), mixed with 3 g of the same vector pCAGGS–MAR GFP (Fig. 1A–C) with different selector cassettes (NeoR, HygroBR, and PuroR) for the single transfection or with 1.5
g plasmid DsRED 1.5 g GFP for the cotransfection, transferred into the nucleofection cuvettes, and transfected with V-24 program (Nucleofector Amaxa).