Lentiviral vectors (LVs) provide a powerful tool for gene transfer into both dividing and non-dividing cells (for a review see Vigna and Naldini 1). They are able to stably transduce primary and terminally differentiated cells such as lymphocytes, hematopoietic stem cells, macrophages, neurons, and hepatocytes from different species, including rodents and primates. Moreover, LVs pseudotyped with the G protein envelope of the vesicular stomatitis virus (VSV-G) can be concentrated to high titers and allow efficient transduction of a wide range of tissues in vivo. 2-4

Pseudotyped LVs are currently produced by transient transfection into 293T cells of a combination of plasmids encoding the required lentiviral packaging functions, the envelope of an unrelated virus that pseudotypes the particle, and the transfer vector (see [26] by Follenzi and Naldini). Different versions of these constructs and their combinations have been described and characterized, from HIV, SIV, and nonprimate lentiviruses. Here we refer to a late-generation vector system that we have contributed to develop from HIV-1 and which has been extensively characterized for performance and biosafety. 5-7 However, the approaches