HIV. Scanning electron micrograph (SEM) of viral vectors based on HIV (human immunodeficiency virus), budding from a 293T cell line. Viruses have evolved elegant strategies to enter target cells and seize the cellular machinery to express viral genes and assemble progeny particles. In the same way, viral vectors represent the most effective means of gene transfer to modify specific cell types or tissues, and can be manipulated to express therapeutic genes. Viral vectors can be used as in vitro tools for biomolecular and gene functional studies, but also to accomplish more demanding tasks such as treating genetic disorders, fighting cancer, driving tissue regeneration and monitoring cell function. Magnification: x6000 when printed at 10 centimetres wide. Specimen courtesy of Yasu Takeuchi Group, Division of Infection and Immunity, UCL.