An ideal cloning vector should possess: (i) an origin of replication (ori) so that it can replicate inside the host, (ii) one or more selectable marker genes (such as antibiotic resistance) to identify transformed cells, and (iii) a single recognition site for restriction enzymes within the marker, so the foreign DNA inserts at a known place.

If a vector has two or more recognition sites for the same enzyme, the vector itself would be fragmented during digestion, complicating the cloning process. Hence multiple sites are not desirable.