Late Replacement Vectors - The region encoding VPI, VP2 and VP3 may be replaced in the vector by DNA insert; such a vector is called late region replacement vector, e.g., SVGT-5 (an improved SV40 vector). A vector of this type is used for infection of host cells in conjunction with another virus, called helper virus, which has the VP1, VP2 and VP3 genes, intact but has a defective large-T gene.

In this case, only those host cells that are infected by both the vector and the helper virus will lyse and produce virions since cells infected by either the vector or the helper alone will not support packaging or replication (respectively) of the virus. This feature is very useful since all the plaques formed on monkey cell monolayers contain the vector.