Adenovirus vectors are the most commonly employed vector for cancer gene therapy. They are also used for gene therapy and as vaccines to express foreign antigens. Related Theory Gene therapy is an experimental technique in which genes are introduced into individual cells or tissues, to cure some hereditary diseases in which a defective allele of a gene is replaced with normal one. It involves the treatment of genetic disease by replacing the defective gene by a normal one, used in the treatment of several diseases such as SCID (Severe combined immunodeficiency), Cystic fibrosis, Sickle cell anaemia.