This research studied recombinant DNA-derived protein expression utilising expression vectors containing IRES sequences to link the gene of interest with the gene encoding selectable marker in mammalian cell cultures. Polycistronic expression constructs utilising internal ribosome entry site (IRES) can link unrelated genes under control of a single promoter. Transient study on the IRESlinked gene expression was performed. It was possible to standardise the level of protein expression to plasmid number by determining the number of free plasmids in the cytoplasm. The expression of a selectable marker when downstream of IRES was reduced in comparison to the monocistronic construct. Importantly when IRES was used, there were no negative effects on recombinant gene expression upstream of IRES. Down-regulating the selectable marker gene expression has been shown to enhance the probability of obtaining highly expressing clones. To investigate the effects of down-regulating fusion metallothionein green fluorescent protein (MTGFP), new constructs were created to combine metal inducible M2.6 promoter to drive the expression of human growth hormone linked to MTGFP by an attenuated IRES. This resulted in less MTGFP expression, reduced survivability and mean fluorescence in the presence of heavy metal. The increased metal sensitivity lengthened the initial selection period using reduced metal concentration in comparison to cells transfected with wildtype MTGFP. FACS can be used to select for resistance conferred by MTGFP despite reduced expression. FACS enrichment and sorting increased the hGH expression, which was correlated with mean fluorescence of the population; therefore fluorescence can be used as an indication of the final recombinant protein expression. Different approaches to isolate suitable clones were also investigated. It is preferable to select the transfected pool in low metal concentration for two weeks, sort for cells of high-fluorescence, and allow for recovery and proliferation. It is then possible to amplify gene expression by culturing the clones in increasing metal, resulting in further improvement of recombinant protein expression.