The number of transformed cells is usually very low compared to the cells that did not take up thevector. Hence, the transformed cells have to be selected somehow. The most common method is based on antibiotic resistance genes that are part of the vector construct. After transformation, the cells can be grown on a rich medium containing the matching antibiotics. On this selective media, only cells containing the plasmid vector are able to multiply and produce colonies.

Antibiotic selection is an important step ofmolecular cloning.