Research initially focused on adeno-associated viruses (AAV) for gene therapy, but their limited integration into human DNA restricted use to non-dividing cells. The discovery of lentiviral vectors, repurposed from the AIDS virus, revolutionized gene therapy by enabling integration into rapidly dividing cells. This advancement expanded treatments beyond rare disorders to complex diseases like cancer, with therapies like Novartis's Kymriah reprogramming immune cells to fight leukemia. New approaches are now exploring in vivo delivery, aiming to make gene therapy more accessible and efficient.