We can use ampicillin resistance gene as

Ampicillin gene can used as a selectable marker in rDNA technology. If a recombinant DNA bearing gene for resistance to an antibiotic (e.g., ampicillin) is transferred into E.coli cells, the host cells become transformed into ampicillin-resistant cells. If we spread the transformed cells on agar plates containing ampicillin, only transformants will grow, untransformed recipient cells will die.