pUC18 is commonly used plasmid cloning vector in E.coli. The molecule is a small double-stranded circle, 2686 base pairs in lenght, and has a high copy number. pUC18 carries a 54 base-pair multiple cloning site polylinker that contains unique sites for 13 different hexanucleotide-specific restriction endonucleases. The molecular weight of pUC18 is 1.75x106 daltons