Genetic engineering is a branch of science that seeks to develop a trait or set of traits in an organism by inserting, or incorporating, a sequence of genetic code that is specific for that trait. Viruses are tiny, nonliving particles that must have a host cell to replicate and flourish. One of the qualities viruses have that makes them attractive to genetic engineering scientists is their ability to attach to and invade specific cells, and incorporate the DNA (and/or RNA) they are carrying into the host cell, where it combines with the host cell's DNA. This invasive quality viruses have provides scientists with a "key" to open the door to the DNA in the cell which they want to modify.
Viruses use a proton called "glycoprotein" as a "lock and key" mechanism to enter specific cells and inject their DNA. A scientist could use a specific virus in DNA technology to target specific cells.