When geneticists use small pieces of DNA to clone a gene and create a GMO, that DNA is called a vector. The vector serves as the carrier for the transfer or insertion of gene(s). Vectors are among the essential tools for gene cloning and are most useful if they also encode some kind of marker gene encoding a bioindicator molecule that can be measured in a bioassay to ensure their insertion, and expression, in the host organism.
In some cases, viruses are used to infect bacteria. These viruses are called bacteriophages, or phage, for short. Retroviruses are excellent vectors for introducing genes into animal cells.Plasmids, circular pieces of DNA, are generally used to introduce foreign DNA into bacterial cells. They often carry antibiotic resistance genes that can be used to test for expression of the plasmid DNA, on antibiotic petri plates. Gene transfer into plant cells is commonly performed using the soil bacterium Agrobacterium tumefaciens, which acts as a vector and inserts a large plasmid into the host cell.