Fig. 1. Transcription map and genome organization of adenovirus and adenovirus-derived gene transfer vectors. A schematic representation of the adenovirus genome is shown at the top of the figure. The length of the linear double-stranded adenovirus genome is approximately 36 kb and is arbitrarily divided into 100 map units. Black boxes at the terminal ends of the genome represent inverted terminal repeats (ITR). Arrows indicate the direction of transcription for the major messenger ribonucleic acids (mRNAs). E1–E4 indicate early transcripts, L1–L5 represent late transcripts, and MLP is the position of the major late promoter. Below are represented the first-, second- and third-generation (or “gutted”) adenovirus vectors. The transcription regions that are deleted are indicated in each vector. (Adapted from Wu Q, Moyana T, Xiang J: Cancer gene therapy by adenovirus-mediated gene transfer. Curr Gene Ther 1:101, 2001)