SV40 Plasmid Vectors - These vectors replicate in monkey cells but do not get packaged into virions. They contain the origin of replication and the large- T encoding gene (large- T gene is not necessary for multiplication in COS cells). Obviously, there is no size limit on such vectors, and some of them are E. coli and monkey shuttle vectors, e.g., pSV2, pSV3, etc.

These vectors produce high copy number per cell. The shuttle vectors are used to propagate the recombinant vector in E. coli, which are then introduced into monkey cells to study the expression of DNA inserts. Plasmid vectors are unstable in monkey cells, and are generally used for transient transfection only.