The expression enhancement by cytomeg-alovirus promoter and different intron A (IA) variants were evaluated in CHO-K1, HepG2, HEK-293 and COS-7 cells by assessing the levels of luciferase activity. This data along with mRNA levels measure-ment indicated that the construct harboring an IA variant with a 200-nucleotide deletion (D200) had the greatest impact on increasing luciferase expression among all constructs evaluated. Based on these results, we redesigned pCMV–IA variants and cloned them into plasmids expressing a humanized antibody. These plasmids were then used to transfect CHO-K1 cells. Production of the antibody was not augmented with the D200 promoter variant. The 600-nucleotide dele-tion (D600) and whole IA promoter variants expressed similar levels of the recombinant protein. These data indicate that the IA-based enhanced expression of transgenes depends on a small region within the intron.