Viral gene therapy. Illustration of adenovirus particles (green) delivering therapeutic DNA (deoxyribonucleic acid) to a cell. Adenoviruses have been investigated as gene therapy vectors since the 1990s. They are common human pathogens that most often cause a mild cold. The viruses used as vectors for gene therapy are modified so as to not cause disease. However, most individualIllustrations immune systems will recognise the virus and destroy it before it gets to deliver its therapeutic DNA and some will have a potentially life-threatening immune reaction the the virus. Despite this adenovirus vectors are still being trialled, particularly for the treatment of cancers, as they are well understood and can infect a wide range of human cells. The first adenovirus vector gene therapy was approved in the USA in 2022 to treat a type of bladder cancer.