Viral vector manufacturing refers to the production of a carrier called a vector which is genetically engineered to deliver the gene because they can deliver the new gene by infecting the cell. Considered as a drug substance, viral vectors are used in gene therapy and vaccinology (vaccines). In the case of cell therapies, the scrutiny for testing for adventitious agents is high. Viruses that are often used as vectors include retroviruses that integrate their genetic material including the new gene in the human cell, and adenoviruses, that introduce their DNA into the nucleus of the cell without letting the DNA to integrate into a chromosome. These viruses are modified so they can't cause disease when used in people.