Genome structures of the first, second, and third generation of adenoviral vectors. Wild-type adenoviral sequences are labelled in black. The localization of the early genes (E1–E4) is represented by arrows. Deletion sites are shown as a thin line or as striped boxes for alternative deletions. ITR: inverted terminal repeats; Ψ: packaging signal (modified from: X. Danthinne [125]).