a) Schematic drawing of the lentiviral vectors used. Abbreviations as in Figure 1; RFP: red fluorescent protein; Luc2: codon-optimized (for homo sapiens) firefly luciferase; H1p: H1 RNA polymerase III promoter; shRFP: small hairpin RNA targeting the RFP transcript; shNT: small hairpin RNA with Non-Targeted control sequence. 