We describe the development of an efficient expression system suitable for the stable expression of recombinant genes in Chinese hamster ovary (CHO) cells using the human interferon β SAR element. The insertion of two copies of the human interferon β SAR element at the 5' and 3' flanking regions of the β-galactosidase reporter gene increased the frequency of β-galactosidase positive colonies by up to 75% and enhanced P-galactosidase expression by 15- to 20-fold after G418 selection or 30- to 40-fold at the initial stage of the MTX selection procedure. Deletion analysis showed that the whole DNA regions of the human interferon P SAR element are required for β-galactosidase expression enhancement. The developed expression system was also highly effective at enhancing the stable expression of two therapeutically important proteins, namely, erythropoietin (EPO) and hepatocyte growth factor (HGF). We isolated stable colonies with expression levels of 47 μ g/10(6) cells/day for EPO and 13 μ g/10(6) cells/day for HGF, suggesting that the developed expression system based on the human SAR element is suitable for expressing high levels of recombinant proteins in CHO cells.