A HeLa cell line that stably expresses DV1 prM and E envelope proteins was then established by transducing HeLa cells with a retroviral vector pCHMWS-prMEopt-IRES-Hygromycin. Hygromycin-selected cells were designated HeLa-prME. Several single colonies of HeLa-prME were used in this study and they showed similar results. The HeLa-prME cell line has already been propagated for more than thirty passages in the presence of hygromycin and has kept a stable prME expression level. Culture of HeLa-prME cells in the absence of hygromycin resulted in a significant reduction of prME expression (data not shown).