A selectable marker is carried by the vector to allow the selection of positively transformed cells.  Antibiotic resistance is often used as marker, an example being the beta-lactamase gene, which confers resistance to  the penicillin group of beta-lactam antibiotics like ampicillin.  Some vectors contain two selectable markers, for example the plasmid pACYC177 has both ampicillin and kanamycin resistance gene. Shuttle vector which is designed to be maintained in two different organisms may also require two selectable markers, although some selectable markers such as resistance to zeocin and hygromycin B are effective in different cell types.  Auxotrophic selection markers that allow an auxotrophic organism to grow in minimal growth medium may also be used; examples of these are LEU2 and URA3 which are used with their corresponding auxotrophic strains of yeast.
Another kind of selectable marker allows for the positive selection of plasmid with cloned gene.  This may involve the use of a gene lethal to the host cells, such as barnase, Ccda, and the parD/parE toxins.  This typically works by disrupting or removing the lethal gene during the cloning process, and unsuccessful clones where the lethal gene still remains intact would kill the host cells, therefore only successful clones are selected.