Illustration of the capsid of adeno-associated virus 9 (AAV9) particles assemmbling. The capsid is the protein shell that encloses the virus's genetic information, in this case single-stranded DNA (deoxyribonucleic acid). Adeno-associated viruses (AAVs) are the smallest known viruses to infect humans, with the capsid measuring only 20 nanometres across. They do not cause diseases, and only provoke a mild immune response. Because they incorporate their genetic material into a specific location within the host's genome, they are being trialled as a vector for gene therapy.