Abstract: Recombinant proteins are of great commercial interest. Yet, most current production methods in mammalian cells involve the time- and labor-consuming step of creating stable cell lines. Production methods based on transient gene expression are advantageous in terms of speed and versatility, yet, thus far, those methods have not shown the specific productivity, batch yield and volumetric yield to be an economic alternative to stable cell lines. The inventors improved on the methodology of transient transfection and achieved commercially relevant yields in terms of specific productivity (exceeding 35 pg per cell per day), batch yield (exceeding 700 mg/l) and volumetric yield.