Adenoviral (Ad) vectors are based on a relatively non-pathogenic virus that causes respiratory infections. The 36 kb linear, double-stranded Ad DNA is packaged in a 100 nm diameter capsid. In first-generation Ad vectors, the early region 1 (E1) gene was deleted to generate a replication-defective vector, and to create space for an inserted gene coding for a marker or therapeutic protein. A cell line that complements the E1 gene deletion allows propagation of the viral vector in cultured cells. These first-generation Ad vectors can accommodate up to approximately 8 kb of insert DNA. In high capacity Ad vectors, the entire Ad vector genome is ‘gutted’ (hence the alternative name, ‘gutted Ad vector’) removing all viral genes and providing 30 kb of insert cloning capacity.