Cells of E. coli that have been transformed by an unmodified pUC18 plasmid form blue colonies. However, if the plasmid contains a foreign DNA insert in its polylinker region, the colonies are colorless because the insert interrupts the protein-coding sequence of the lacZ gene and no functional ␤-galactosidase is produced. Bacteria that have failed to take up any plasmid are also colorless due to the absence of ␤-galactosidase, but these cells can be excluded by adding the antibiotic ampicillin to the growth medium (the plasmid includes the gene amp R , which confers ampicillin resistance). Thus, successfully transformed cells