Vectors also incorporate a selectable marker, a gene which allows host cells containing the vector to be selected from amongst those which do not, usually by conferring resistance to a toxin, or enabling their survival under certain growth conditions.

最初的大肠杆菌载体是染色体外的环形质粒，并且相当数量的噬菌体（感染细菌的病毒）也被用在大肠杆菌中。