Adenovirus early region 4 (E4) regulates processes in infected cells that include viral late gene expression, nonhomologous end joining, responses to DNA damage, and apoptosis. E4 is essential for viral growth in most cell lines. In this chapter, the current knowledge of the functions of six E4 products is summarized briefly. Protocols are presented for manipulation of E4, incorporation of E4 mutations into the viral genome, and growth of E4 mutants on complementing cell lines. A compilation of the described E4-complementing cell lines is included.
