This 3rd generation LV packaging plasmid includes gag, coding for the virion main structural proteins; pol, responsible for the retrovirus-specific enzymes; and RRE, a binding site for the Rev protein which facilitates export of the RNA from the nucleus. This plasmid does not include Rev. The third generation packaging system offers maximal biosafety but is more cumbersome, as it involves the transfection of four different plasmids in the producer cells.