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The first generation LV packaging system encompasses all HIV-1 genes besides the envelope.
The second generation LV packaging system is additionally deleted in all viral auxilliary genes,
i.e. vpr, vif, vpu and nef. Examples: pCMV-dR8.91, pCMV-dR8.74, psPAX2
The third generation LV packaging system comprises only gag, coding for the virion main structural
proteins, pol, responsible for the retrovirus-specific enzymes, and rev, which encodes a post-transcriptional
regulator necessary for efficient gag and pol expression. a cDNA encoding rev is provided on a separate plasmid. The third generation packaging system offers maximal
biosafety but is more cumbersome, involving the transfection of four different plasmids in the producer cells.
Example of the 3rd generation packaging combo: pMDL g/p RRE + pRSV-Rev.
