A great variety of viral and nonviral expression systems has been developed and assessed for their ability to transfer genes into somatic cells. In particular, retroviral and adenoviral mediated gene transfer has been extensively studied and improved at least because of their capability to efficiently infect the targeted cells. However, the lack of cell type specificity of viral and nonviral vectors still represents one major obstacle for appropriate and controlled expression of foreign genes. Many attempts have been made to achieve efficient gene delivery by targeting, e.g., retroviral integration via modifications in viral envelope protein sequences, using antibodies as specific mediators in viral infection and pseudotyped viruses, and so on (1 , 2 , 3 , 4 ).
