Gene therapy can be defined as the transfer of genetic material into a cell for therapeutic purposes.Cytosine deaminase (CD) transferred into tumor cells by an adenoviral vector, can convert the antifungal drug 11uorocytosine (5-FC) to the antimetabolite 5-fluorouracil (5-FU),which kills not only the transfected tumor cells but also their neighbors by the so-called bystander effect.