A cloning vector is a kind of recombinant DNA molecule that carries foreign DNA fragments and can be stably maintained in an organism, where it can be replicated and/or expressed. It is used as a vehicle to artificially transfer foreign genetic material into another cell. Four major types of vectors are being used: plasmids, viral vectors, cosmids and artificial chromosomes, of which the most commonly used one is plasmids.
Vectors can be specially designed for cloning or for other purposes, such as transcription and protein expression. Transcription vectors lack the sequence necessary for polyadenylation and termination, therefore they cannot be expressed but can only be used for transcription, for example for in vitro mRNA production. Expression vectors, in contrast, generally have a promoter sequence that drives the expression of the transgene.