Fig. 3. Genome structure of adenovirus serotype 5 and different generations of adenovirus vectors.

The adenovirus genome encodes early (E) and late (L) genes in which the E1A protein, an immediate early protein, induces the expression of delayed early proteins, including E1B, E2A, E2B, E3, and E4. In first-generation adenovirus vectors, the E1 and E3 regions were deleted, and in the second generation, E1, E2, E3, and/or E4 were deleted. Helper-dependent adenovirus vectors only contain inverted terminal repeats (ITRs) and packaging signals. Oncolytic vectors are another class of adenovirus vectors. For example, Delta-24 vectors with a deletion in the E1A region replicate in and lyse tumor cells with a defect in the pRB pathway.