Glycosylation is a major post-translational modification of proteins that involvesattachment of glycan/oligosaccharide chains to the protein (1). In producingrecombinant glycoproteins it is essential to exactly replicate the native glycosylationpattern in order to ensure proper functional activity of the glycoprotein. For thisreason the mammalian Chinese Hamster Ovary (CHO) cell line has generally beenthe host of choice. In order to evaluate the adopted dual vector expression system,we have chosen to express the glycoprotein bovine FSH. This glycoprotein isdifficult to purify in substantial quantities from bovine pituitaries. A commercialform is available, however, it is frequently contaminated by other hormones such aslutenizing hormone, which it is believed, leads to some of the variability that isencountered with the superovulation procedure for cows. The aim of this project isto develop a system for expression of recombinant FSH by improving upon currentCHO expression systems. The gene promoter is an obvious feature whereexpression levels can be controlled and/or maximised (2). An important aspect ofthis project was ultimately to replace the existing viral promoters with strongconstitutive promoters native to CHO cells (i.e. homologous).