Cancer cells adapt to chronic stress in the tumor microenvironment by inducing the expression of GRP78/BiP, a major endoplasmic reticulum chaperone with Ca²⁺-binding and antiapoptotic properties. GRP78 promotes tumor proliferation, survival, metastasis, and resistance to a wide variety of therapies. Thus, GRP78 expression may serve as a biomarker for tumor behavior and treatment response. Combination therapy suppressing GRP78 expression may represent a novel approach toward eradication of residual tumors. Furthermore, the recent discovery of GRP78 on the cell surface of cancer cells but not in normal tissues suggests that targeted therapy against cancer via surface GRP78 may be feasible. [Cancer Res 2007;67(8):3496–9]