Map of adenovirus serotype 5 genome and different generations of adenoviral vectors. Early transcripts are represented by E1–E4 regions and late transcripts are represented by L1–L5 regions. MLP: major late promoter; 
: packaging signal.
First-generation system:
– Two adenoviral early genes, E1 and E3, are deleted
– Two adenoviral early genes, E1 and E3, are deleted
– Transgene expression istransient and the vector is quickly lost in cells
– Causes high cytotoxicity or immune response
– Causes high cytotoxicity or immune response
– A single step of homologous recombination between the vector and host genome sequence may give rise to RCA generation
Second-generation system:
– Additional essential genes, E2 and E4 are deleted in addition to E1 and E3
– Transgene is expressed at a higher lever for a longer duration
– Diminished cytotoxicity or immune response
– Deletion of E4 and E2 genes which are essential for adenoviral growth and replication further reduces the potential RCA generation