To produce the retroviral vector for delivery of the prME-opt gene into HeLa cells, the pCHMWS-prME-opt-IRES-Hygromycin, pcDNA-VSV-G and p8.71 (modified HIV provirus coding for gag and polymerase) plasmids were co-transfected into 293T cells. The cell supernatant containing infectious VSV-G-pseudotyped retroviral particles was harvested 48 hours post-transfection and used to infect HeLa cells. Two days after infection, cells were selected in culture medium containing 500 mg/ml of hygromycin for two weeks. Selected cells (HeLa-prME) were tested by flow cytometry for E expression and were maintained in DMEM +10% FBS +500 mg/ml of hygromycin.