| Feature | 2nd Generation | 3rd Generation |
|---|---|---|
| Transfer Plasmid | Can be packaged ONLY by a second generation packaging system that includes TAT | Can be packaged by both 2nd and 3rd generation packaging systems |
| Packaging Plasmid | All on one plasmid: Gag, Pol, Rev, Tat | Two plasmids: one encoding Gag and Pol and another encoding Rev |
| Envelope Plasmid | Interchangeable: usually encodes for VSV-G | Interchangeable: usually encodes for VSV-G |
| Safety | Safe. Replication incompetent: Uses 3 separate plasmids encoding various HIV genes. | Safer. Replication incompetent and always SIN: Uses 4 plasmids instead of 3 and eliminates the requirement for Tat. |
| LTR Viral Promoter | Wild type | Hybrid: 5’LTR is partially deleted and fused to a heterologous enhancer/promoter such as CMV or RSV |
