The invention relates to a method for producing a transformed eukaryotic host cell which expresses a recombinant polypeptide of interest which method comprises introducing into a eukaryotic host cell: (a) a first polynucleotide vector which comprises (i) a first nucleotide sequence which encodes a recombinant polypeptide of interest, and (ii) a second nucleotide sequence encoding a selectable marker, which second nucleotide sequence is amplified when the host cell is contacted with a selection agent and (b) a second polynucleotide vector having essentially the same nucleotide sequence as the first polynucleotide vector except that the second nucleotide sequence is replaced with a third nucleotide sequence which encodes a different selectable marker the first polynucleotide vector and second polynucleotide vector being stably integrated into the genome of the host cell.
