Retrovirus: Classic, can integrate into the genome but with low transduction efficiency. They are useful for gene transfer and protein expression in cells that have low transfection efficiency with other transfection reagents. Lentivirus: Can integrate into the genome with relatively high transduction efficiency and they are very useful for cells that have low transfection efficiency with other transfection reagents. No special competent cells required, as they are stable plasmids. Lentiviruses are a powerful tool for stable gene transfer to both dividing and non-dividing cells in vitro and in vivo. Adenovirus: Only work transiently (about 7 days) but have almost 100% transduction efficiency. Adenoviruses can infect a broad range of cell types with the highest efficiency and infection is not dependent on active host cell division. A second key feature is that high virus titers and high-level gene expression can be obtained in most mammalian cells.

For lentiviruses and retroviruses, they are measured in CFU/ml (colony-forming units per millilitre). Transduction with lentiviruses and retroviruses can cause the formation of colonies, which can be quantified for concentration. For AAV the titer is measured as genome copies per mL (GC/mL). Adenoviruses are measured as PFU/ml (plaque-forming units per millilitre). Transduction with adenoviruses will kill packaging cells, forming plaques in the process for quantification. The concentration for all three types of viruses can also be classified as IU/ml (Infectious Units/ml). Ultimately, the units refers to the viral particles and different units reflect the different assays involved.

You can observe transduction efficiency from 48 hours up to 5 days after infection.