Based on our previous findings [14, 15] , a comparison of luc activity between SV/luc and Ad/luc vectors was performed. As expected, luc expression was lower in the 2CLP-SV/luc than in the 2CLP-Ad/luc group (Figure 2c,  2d) , because of the natural tropism of adenoviral vectors for the pulmonary epithelium [14] . Another factor may have been the higher titer of the adenoviruses used, a total of 10 9 IU/ml adenoviral vector as compared to 1,25 × 10 8 IU/ml of the SV40 vector.