The plasmid pUC18 contains an antibiotic resistance gene to screen for
host cells that have taken up vectors.
A polylinker region contains many restriction enzyme recognition sequences.
The polylinker is inserted into a lacZ gene, whose gene product β-galactosidase can metabolize Xgal supplied in the medium; bacteria with wild type lacZ produce blue colonies.
Recombination within the polylinker disrupts lacZ function, resulting in white colonies.