viral vector, in molecular biology, a tool used to deliver genetic material to cells. Viruses are useful delivery tools, owing to their efficiency in locating and entering target cells. In addition, they are generally safe to use, because all viral genes are taken out and the remaining vector is then modified to transport and deliver only the specific genetic material of interest.

Different types of viruses may be used as vectors, including adenoviruses, adeno-associated viruses, baculoviruses, herpes simplex viruses, poxviruses, and retroviruses. These vectors either become integrated into host cell DNA or, within host cells, temporarily express the genes they carry. Gene expression efficiency, ease of generation, and factors such as stability and potential toxicity determine the selection of vector.

The major applications of viral vectors are in basic research and in the development of vaccines and gene therapy. In many instances, cells targeted by viral vectors are manipulated to express genes that have potential therapeutic effects. For example, viral vectors may be used to transfer the cystic fibrosis gene (CFTR) into epithelial cells in the airway as a form of gene therapy for patients with cystic fibrosis.