Abstract: A method is provided for use in producing a selected protein in mammalian cells, and to cDNA molecules useful in the method, and fusion proteins produced from expression of the cDNA. In the method, cDNA encoding a fusion protein that includes a mammalian DHFR and the selected protein is introduced into mammalian cells such that it is expressed. The naturally occurring repression of DHFR translation is overcome by treatment of the cells with a folate or antifolate or similar composition. The relief from this repression extends to the selected protein which is the second part of the expressed fusion, such that the treatment results in controllable and enhanced production of the selected protein.