Gene therapy uses vectors like viruses, human artificial chromosomes, and bone marrow cells to deliver therapeutic genes. Viruses commonly used include retroviruses, adenoviruses, and adeno-associated viruses. Non-viral methods include naked DNA, lipoplexes, and DNA-molecular conjugates. Gene therapy approaches for cancer include restoring tumor suppressor genes, inactivating oncogenes, suicide gene therapy, and tagging cancer cells. Limitations of gene therapy include only being able to treat single gene defects currently.