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
   b-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.