Though there are several virus delivery systems available today for gene therapy, lentivirus is considered one of the best methods.  Several features make lentivirus preferable especially the ability to infect both dividing and non-dividing cells.  Lentiviral vectors can integrate their genetic cargo directly into the chromosome of the target cell but do not transfer sequences that encode for proteins derived from the packaging virus.  This is key to preventing an immune response to the cells containing the transfer gene.  Finally, lentivirus vectors can be pseudotyped to infect specific or a broad range of targets.