Abstract: The effects of endogenous XBP1 (X-box binding protein 1) overexpression on HBsAg secretion in CHO cells were first tested. However, no improvement was observed with DMSO. But HBsAg secretion was significantly enhanced with DTT, an inducer of ER stress response. Further analysis showed that DMSO did not trigger the ER stress response. The function of XBP1 in facilitating protein secretion depended on whether the protein expression resulted in secretory saturation in ER. On the contrary, the presence of DMSO was able to improve HBsAg folding and facilitate it passing through ER, and caused secretory saturation in other organelle of secretory pathway.