Transcription map of human adenovirus serotypes 2 and 5. (Top) The viral genome (∼36 kb) is represented as a horizontal line with map units (m.u.). Transcription units are shown as arrows indicating direction of transcription. There are four early transcription units (E1–E4) and five families of late RNAs (L1–L5), which are the alternately spliced products of a common precursor expressed from a single major late promoter (MLP). The termini of the double-stranded genome consist of 103-bp inverted repeats (ITR) that are involved in viral DNA replication. Signals for encapsidation of the viral DNA during assembly of viral particles are located 190–380 nt from the left-hand end of the viral genome. (Bottom) Diagrams of the structures of three generations of adenovirus vectors (first, second, and gutless). (Downward arrows) Viral regions that are deleted in first- and second-generation vectors. All second-generation vectors lack early regions E1 and E3; some vectors also lack early regions E2A and E4. The only viral DNA sequences retained in gutless adenovirus vectors are the ITR and packaging signals. (Redrawn from Wu et al. 2001, with permission from Bentham Science Publishers Ltd.)