Adenovirus is a non-enveloped, 90-100 nm diameter virus presenting icosahedral symmetry (Figure1A). Human adenovirus, containing a linear, double-stranded DNA genome, which is approximately 36kb, is wrapped in a histone-like protein and has two inverted terminal repeats (ITRs) of 50-200 bp, which act as origins of replication, accompanied by a complex series of splicing. Adenovirus transcription is a two-phase event, early and late, occurring before and after viral DNA replication, respectively (Figure1B). The early transcribed regions are E1A, E1B, E2A, E2B, E3, and E4, which are successively transcribed early in the viral reproductive cycle. The proteins coded for by genes within these transcription units are mostly involved in regulation of viral transcription, in replication of viral DNA, and suppression of the host response to infection. The late transcribed genes are L1-L5, which are transcribed later in the viral reproductive cycle, and encode mostly for proteins that make up components of the viral capsid or are involved in assembly of the capsid. The genes in the adenovirus and their functions are given in the following table 3.

Figure1. Schematic of the adenovirus genome and adenovirus-based vectors. (A) Capsid crystal structure. (B) Adenovirus gene map. Top panel: A simplified map of the adenovirus serotype 5 genome showing the early genes (E1–E4) and the region from which the major late transcript is produced. Middle panel: General structure of an early region 1 (E1)-deleted Ad vector. Bottom panel: General structure of a helper-dependent Ad vector [15].