A method for controlled production of a desired mature protein in a vertebrate host cell through the use of a polycistronic expression vector, which contains sequences coding for the desired protein and a secondary protein, is described. Both coding sequences are governed by the same promoter and are separated by translational stop and start signal codons. The expression of the secondary sequence effects control over the expression of the sequence coding for the desired protein, and the secondary protein functions as a marker for selection of transfected cells.