Gene therapy describes the pharmaceutical use of DNA, which directly affects the patient's cells. It is often used to treat genetic conditions by delivering DNA that contains a gene that is missing in the patient. Once delivered, the "foreign" DNA is used by the patient's cells to produce any missing proteins.
Originally conceived in 1972, it was not tested experimentally until the 1990s. Even then, it did not appear to provide meaningful results. However, since 2006, many forms of gene therapy have shown great promise in clinical trials and promise new treatments for diseases ranging from hemophilia to Parkinson's disease.