Figure 1.

The scheme of the wild type serotype 5 adenovirus genome. The genome consists of four early transcription elements (E1, E2, E3, and E4) and five late expression genes (L1–L5) associated with adenoviral particle assembly. The E1 gene is required for the activation of the transcription of early genes; the E2 gene is required for virus DNA replication; the E3 gene is required for the modulation and evasion of the host's immune response, prevention of untimely cell death through apoptosis, and efficient cell lysis once new particle assembly is complete; the E4 gene is involved in virus RNA metabolism and transport, preferential downregulation of host-cell protein synthesis, and enhancement of virus DNA replication. The deletion of both the E1 and E3 genes can accommodate up to 7.5 kb of foreign DNA and is commonly used in gene therapy. E4 gene deletion further reduces induction of vector-specific immune responses and minimizes the outgrowth of replication-competent viruses in packaging cell lines.