An expression vector for eukaryotic cells comprising a gene which codes for a protein of interest, functionally linked to a hamster-ubiquitin/S27a-promoter and a gene which codes for a fluorescent protein. Preferably the expression vector also contains an amplifiable selectable marker gene. The invention also describes host cells, preferably mammalian cells, which have been transfected with the expression vector, processes for producing heterologous gene products and a method of selecting high-producing cells.