Gene therapy. Conceptual computer artwork representing gene therapy, showing a syringe being used to inject genetic material into a chromosome. Chromosomes, found in cell nuclei, contain the genetic material DNA (deoxyribonucleic acid). Gene therapy involves new genes being used to replace defective ones. The first approved gene therapy took place in 1990. Since then, a variety of methods and diseases have been investigated and several pioneering treatments attempted. The usual method is viral delivery, where viruses are used to insert new genes into a patient's genome.