To determine the role of GRP78 in endogenous tumor growth, the GRP78+/−mice were crossed with the transgenic mice (MMTV-PyVT) expressing the polyoma middle T oncogene (PyMT or PyT) driven by the murine mammary tumor viral promoter [118], similar to those used by Gong et al. (see [63] and Section 3.1.2). Here, HSPA5 is linked to breast cancer.