FIG. 2.

Determination of Cre activity in PER.tTA.Cre cell clones by titration of Ad.floxedΨ. Ad.Ψ is a first-generation Ad vector with E1 deleted and with the same genetic composition as Ad.floxedΨ (see Fig. 1A) except that it contains a wild-type nonfloxed packaging signal. Six million PER.tTA.Cre76 and PER.tTA.Cre43 cells were infected (+) for 24 h with Ad.floxedΨ or Ad.Ψ at an MOI of 5 IU/cell. The amounts of Ad vectors with E1 deleted produced in both cell lines were determined by endpoint titrations on E1-complementing 911 cells (8). The numbers above the bars indicate the average titers from three independent infection experiments.