HIV infected cell and DNA, conceptual composite image. Scanning electron microscopy (SEM) and computer generated image (CGI) of DNA (deoxyribonucleic acid) and an HIV (human immunodeficiency virus) infected cell. When HIV infects a cell, it first attaches to and fuses with the host cell. Then the enzyme reverse transcriptase converts the viral RNA (ribonucleic acid) to DNA and the virus uses the host cell's machinery to replicate itself. 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.