Fig. 3.

Schematic of HIV-1 and the evolution of lentiviral packaging systems. Lentiviral system derived from HIV-1 has been advanced in iterative cycles toward safety improvement and reduction in the viral replicative competency. During this process, the HIV-specific accessory genes that contributing to pathogenesis but nonessential for viral assembly are removed, then tat gene is also removed when the wild type 5′ LTR has been modified. Pseudotype env gene from vesicular stomatitis virus glycoprotein is applied to improve stability and increase tropism. RRE: Rev responsive element; cPPT/CTS: central polypurine tract/central termination sequence; WPRE: woodchuck post-transcriptional regulatory element.