Delivery of genetic material via transfection or transduction: : delivery problem of vectors :Genes can either be introduced as episomes, by transfection, but the process is inefficient and the effects transient, or by viral delivery (transduction). In this case the gene can integrate in the genome, and therefore produce stable effect, and the efficiency is high, but the risk of unwanted effects is also higher. Integrating viruses used nowadays are lentiviruses (such as HIV) or retroviruses: they are very efficient and permanent but can be mutagenic. Non integrating viruses, which are DNA viruses (adenoviruses, or adeno-associated viruses, AAVs), allow higher expression of the transgene and have fewer safety concerns, although they can cause an immune response. In vivo gene therapy delivers the transgene directly to the cells in the patient, whereas ex vivo gene therapy requires the extraction of cells form the patient, their culturing, gene modification, and reintroduction in the patient. The latter method can employ selection and therefore the efficiency of delivery is less of an issue.