COS Cell Expression.

COS-1 cells were created by transforming an established line of monkey epithelial cells, CV-1, with a defective mutant of SV40 (1). The SV40 mutant used carried a small deletion within the origin of replication and, although this construct transformed CV-1 cells, which are permissive for lytic growth of SV40, no infectious virus was produced after prolonged culture. One transformed cell line, COS-1, was fully characterized and found to contain the complete early region of the SV40 genome. COS-1 cells express nuclear large T and all proteins necessary for replication of appropriate circular genomes. This was first demonstrated by showing that COS-1 cells could support the replication of early region mutants of SV40. More important, however, the introduction of any plasmid containing an SV40 origin of replication into COS-1 cells results in rapid replication of the plasmid to high copy number. Coincidently, of course, the transfected cells will express any gene on the plasmid that is driven by a suitable eukaryotic promoter. The combined effect of these phenomena is transient high-level expression of the encoded protein.