Fig. 3. Propagation of gutted adenoviral vector constructs. This diagram is a schema for the propagation of gutted vectors. The helper phage is E1 deleted and must be propagated in a cell line (293) that expresses E1. The helper phage contains a packaging signal (Ψ) that is flanked by loxP recombination target sites. The gutted vector construct contains the flanking inverted terminal repeats, the foreign DNA that is driven by a viral promoter, and a stuffer fragment. The 293Cre cell line expresses the bacteriophage P1 Cre recombinase. When the 293Cre cell line is concomitantly infected with the helper phage and the gutted vector construct, the helper phage packages the gutted vector construct. Meanwhile, the helper phage undergoes loss of the packaging signal (Ψ) through recombination of the loxP sites with Cre recombinase and cannot be packaged. This process allows amplification and packaging of the gutted vector construct by serial passage, while minimizing helper phage contamination of the prepared virus stock. (Adapted from Parks RJ, Chen L, Anton M et al: A helper-dependent adenovirus vector system: Removal of helper virus by Cre-mediated excision of the viral packaging signal. Proc Natl Acad Sci U S A 93:13565, 1996)