How Gene Therapies Are Made Genes In gene replacement and gene addition therapies, a new, working copy of a human gene is delivered to the cell, allowing the body to make the protein that is missing, malfunctioning, in short-supply, or may otherwise be helpful to fight off disease. This new gene is created in a laboratory and is specific to the disease being treated. That is, scientists work to discover which gene needs to be added or replaced and figure out how to create the new, working gene. This is one reason why a single gene replacement therapy can take many years—even decades—to research and produce. Nuclease Nucleases are naturally occurring enzymes that can break the bonds in DNA, essentially cutting it like scissors. In gene editing, nucleases are programmed by scientists to be able to target exact sequences of DNA, which then trigger DNA repair. Sometimes information is included on how to rebuild the DNA to override the harmful code that led to the disease. Vectors Vectors are the delivery vehicles used to carry new genes, synthetic RNA, or nucleases into the right cells inside the body. These delivery vehicles are typically made from naturally occurring viruses. Viruses are used because they are very good at getting inside of cells. However, scientists remove the DNA of the virus so that it won’t make people sick when used as a vector. Viruses commonly studied for use as viral vectors in gene therapy include retroviruses, adenoviruses, adeno-associated viruses (AAVs), and lentiviruses. AAVs have been approved for use in gene therapy. AAVs are not to be confused with adenoviruses, a similar group of viruses, some of which can cause illness. AAVs were first discovered in 1965. These vectors are used because they can get inside many different types of cells. AAVs are not known to cause illness in people and have demonstrated safety in clinical trials. Other viruses are also being researched as possible vectors for use in gene replacement. Because each of these viruses has a unique set of characteristics, some may be better suited than others to use as vectors for particular diseases. Integrating viral vectors could be important for genetic diseases that occur in cells that frequently copy themselves—that way, the new cells can keep producing the genetic information with the intended effect. Nonintegrating viral vectors could be better suited for use in cells that don't copy themselves often. They have a reduced risk of genotoxicity and could allow the newly introduced gene to work as intended for a long period of time. Current gene therapies are the cumulative result of nearly a century of genetics research. RNA Ribonucleic acid (RNA) can work as a messenger, carrying instructions from DNA that controls the production of proteins. Gene therapy strategies targeting RNA are being studied, including gene silencing by RNA interference (RNAi).