Gene therapy is the introduction of new genetic material into a cell as a means to correct a known mutation that causes a disease. Viral vectors are viruses with new or modified genomes that remove the virus' pathogenicity. By replacing pieces of a viral genome with a known genetic sequence, harnessing the viral lifecycle allows the capability to carry the new viral sequence across the cell membrane and bring the modified genome in contact with a defective cellular genome. In order to be successful at transforming the defective genome, viral vectors must overcome physical barriers and immune responses.