Viral gene therapy, computer model. A strand of DNA (deoxyribonucleic acid) is being delivered by a virus, illustrating the concept of gene therapy using viruses. The two objects are drawn at different scales. The geometric structure is the capsid (protein shell) of the virus. The spiral strand is DNA's double helix molecular structure. 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. Viruses are the usual delivery method for gene therapy.