Transformation of competent cells is an inefficient procedure, So some way must befound to distinguish a cell that has taken up aplasmid from the many thousands that have not been transformed. The answer is to make use of a selectable marker carried by the plasmid. A selectable marker is simply a gene thatprovides a transformed cell with a newcharacteristic. non-transformants are not with a new characteristic.A good example of a selectable maker is the ampicillin resistance gene of pBR322. After a transformation experiment with pBR322, onlythose E. coli cells that have taken up a plasmid are ampR tetR and able to form colonies on anagar medium that contains ampicillin or tetracycline. Non-transformants, which are still ampStetS, do not produce colonies on the selective medium. Transformants and non-transformants are therefore easily distinguished.