A method is described for the expression of heterologous polypeptides in rodent cells. The method comprises the oriP/EBNA-1 episomal replication and maintenance system of the Epstein-Barr-Virus (EBV). With the stable integration of an EBNA-1-protein expression cassette under the control of a promoter into the genome of a rodent cell an EBNA-1-protein expression in the cells was obtained. The heterologous protein is expressed from an episome comprising an EBV origin of replication and a functional expression cassette of said heterologous protein. The invention further comprises transformed rodent cell lines, a method for the production of a heterologous protein in said cell lines and a kit for the construction of said cell lines.