Early Replacement Vectors - Alternatively, the essential genes missing from the vector may be present within the genome of host cells. For example, COS (CV-l, origin of SV40; CV-l is a monkey cell line) cell line of African green monkey kidney cell cultures contains in its genome the gene for large- T of SV 40.

Therefore, a vector having the origin of replication and genes for VP1, VP2 and VP3 will replicate and produce virions in COS cell line cells. In such a case, no helper virus is required. Since in such a vector the early genes (large-T gene) are replaced by the DNA insert, it is called early region replacement vector.