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.