The present invention is within the field of industrial protein production. The invention provides a novel expression system using dehydroorotate dehydrogenase (DHODH) as a selection marker in combination with leflunomide or a metabolite thereof, notably for use in mammalian cell lines. Expression vectors encoding DHODH, cell lines comprising said vectors and methods of producing recombinant proteins are also provided.

The present invention concerns a cell line comprising an endogenous dehydroorotate dehydrogenase (DHODH) gene which is partially or fully inactivated, and its use for producing recombinant proteins.