Viral expression plasmids for production of proteins, antibodies, enzymes, virus-like particles and for use in cell-based assays

An expression system to provide extended episomal replication of hybrid plasmid in eukaryotic cell lines, said system comprising:

• a) a vector comprising a polyoma virus core origin according to SEQ ID NO:2, an FR element of EBV, a selection marker, and a gene of interest operably linked within an expression cassette expressing the gene in a eukaryotic cell;
• and
• b) a compatible mammalian cell line, where the vector is expressed, said cell line constitutively expressing EBV EBNA1 protein and PyV LT protein either in presence or in absence of selective pressure.