Figure 1.

Generation of Ad-based vectors complemented by a helper virus containing a loxP-flanked packaging signal. Infection of Cre-expressing 293 cells with the helper virus results in excision of the viral packaging signal, rendering the helper virus DNA unpackagable. The helper virus will then provide all of the functions necessary in trans for replication and packaging of an Ad vector containing appropriate cis-acting elements [i.e., viral packaging signal (ψ⁺) and inverted terminal repeats]. Since all functions necessary for virion formation are provided by the helper virus, the majority of the Ad sequences contained in the vector can be replaced by a foreign gene and other non-Ad sequences (stuffer). The titer of the vector can be increased by serial passage in helper virus-infected 293Cre cells.