In molecular cloning, vectors are DNA molecules used to artificially deliver foreign genetic material into another cell. The four major types of vectors are plasmids, viral vectors, cosmids, and artificial chromosomes, with the most commonly used of these being plasmids. Vectors may be used for cloning, and certain vectors are specifically tailored to that exact purpose, while others may be designed specifically for transcription and protein expression. Simpler transcription vectors are used to amplify their inserted genetic material.