Medical Genetics A. Some of the different types of viruses used as gene therapy vectors (1) Retroviruses a class of viruses that can create double-stranded DNA copies of their RNA genomes. These copies of its genome can be integrated into the chromosomes of host cells. Human immunodeficiency virus HIV is a retrovirusMedical Genetics A. Some of the different types of viruses used as gene therapy vectors (1) Retroviruses A class of viruses that can create double-stranded DNA copies of their RNA genomes. These copies of its genome can be integrated into the chromosomes of host cells. Human immunodeficiency virus (HIV) is a retrovirus