In the past, researchers developing gene therapy applications used replication-defective human Adenovirus 5 (Ad5) as a vector for delivering DNA sequences, almost exclusively. Ad5 vectors are typically rendered replication defective by the deletion of E1a gene sequences. A complementing cell line containing the E1a gene makes it possible to produce Ad5 vectors in large scale. Of the various cell lines that have been constructed for the purpose of high-titer Ad5 production, HEK293 cells and PER.C6 cells are the most widely used…