A replication-defective adenoviral-CMV vector that encodes a wild-type p53 gene. Ad5CMV-p53 induces tumor cells that have been transfected with the vector to produce wild-type p53, a tumor suppressor gene that is deleted or mutated in a significant number of cancers. In transfected tumor cells, the wild-type p-53 gene product exerts an antitumor effect by blocking cell cycle progression at the G1/S regulation point, activating DNA repair proteins in the presence of DNA damage, and initiating apoptosis when DNA damage is irreparable.