The Lentiviral system is very effective at delivering genetic material to whole model organisms and almost all mammalian cells, including non-dividing cells, cells that are not active or growing, and difficult to transfect cells such as neurons, primary cells and stem cells. The efficiency of lentiviral transduction is close to 95% in most systems, which makes the lentiviral vector ideal as an expression vector system. Therefore the system has an important advantage over other existing gene transfer methods such as high efficiency of gene delivery to virtually all cell types and whole model organisms. Besides lentiviral/retroviral expression vectors could contain the sequence features and elements allowing efficient packaging, transduction and stable integration into genomic DNA of target cells, and enabling high levels of expression. Furthermore, with the novel self-inactivation and no unwanted viral replication capacities of current vector systems, transduced cells may be perevented to produce additional viral particles since these cells do not contain the genes needed to produce the viral capsid. Furthermore, upon integration into target cell's genome, the 5' LTR promoter is inactivated, which prevents replication of the viral sequence.