Infection assay. To test whether the spacers increase the infection efficiency, hAd5CMV-GFPΔpIX viruses were propagated in 911 cells expressing the pIX.RGD variants. The capacity of the resulting viruses to transduce cells independent of CAR was tested by exposing CAR-negative Eoma cells to the CsCl purified viruses at 1,000 particles/cell. At 48 h postinfection, the cells were evaluated for GFP expression. (A to F) hAd5CMV-GFPΔpIX with wt.pIX (A), pIX.RGD (B), pIX.flag.RGD (C), pIX.flag.30.RGD (D), pIX.flag.45.RGD (E), or pIX.flag.75.RGD (F). To show that hAd5CMV-GFPΔpIX with wt.pIX (G) and hAd5CMV-GFPΔpIX bearing pIX.flag.75.RGD (H) have equivalent infectious particle numbers, HeLa cells were infected with the corresponding viruses.