A good plasmid vector in recombinant DNA (rDNA) technology should have:
(I) Origin of replication (Ori): Ensures replication inside host cells.
(II) Selectable marker genes: Help identify transformed cells (e.g., antibiotic resistance genes).
(III) Unique restriction sites: Allow insertion of foreign DNA without cutting the plasmid into multiple fragments.
(IV) Ability to multiply and express in a heterologous host.
If a plasmid lacks unique restriction sites, it becomes difficult to insert the gene of interest properly - this is an undesirable characteristic.

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