Comparison of one and two month post transfection stability studies with serum free suspension adapted BHK21 cells. Suspension adapted BHK21 cells were transfected with equimolar amounts of the UCOE plasmids listed in figure 1. These cells were cultured under hygromycin selection for one and two months. The levels of BDD activity (A) and relative BDD copy numbers were measured (B). Data was statistically analyzed using 't' test to detect significant differences between the 3.2 and the 1.5 UCOE containing expression constructs (*: p < 0.05 or more).