The major characteristics of a vector are:

First, a vector should be small in size for easy incorporation into the host cell, yet be able to integrate a large size of the insert. This ensures efficient transfer and accommodation of the foreign DNA.

Second, the vector must have an origin of replication (ori) so that it is capable of autonomous replication inside the host organism. This allows the vector to multiply along with the inserted gene.

Third, the vector needs to have unique restriction sites. If it possesses too many restriction sites, it would be fragmented into several pieces during the cloning process.

Finally, the vector requires at least one selectable marker to screen out transformants. Examples include antibiotic resistance genes such as tetracycline ($tet^{R}$) and ampicillin ($amp^{R}$).