The potential for replacing a mutant gene in cells that are most severely affected and thereby causing a disease has long been desired. An alternative is inserting a normal copy of the defective gene in an organ that can produce the defective protein. After considerable research in animals, recent success in humans has shown promise in a few, serious genetic diseases. For example, an adeno-associated virus containing a normal copy of clotting factor IX was inserted into the liver of patients with hemophilia B and largely eliminated the need for intravenous infusions of the missing factor. In a different approach, a normal copy of a mutant gene that causes retinal dystrophy was injected into the retina of a patient resulting in restored vision. Numerous preclinical and clinical trials for other genetic disorders are underway.
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