GRP78 is elevated in antiestrogen-resistant breast cancer. A, immunohistochemical staining of human ER+, triple-negative, and HER2-amplified breast tumors and their surrounding normal tissue. Sections were stained using a GRP78-specific antibody; tissue sections incubated with a nonspecific mouse IgG were used as a negative control. B, quantification of GRP78 expression in human ER+, triple-negative (TN), and HER2-amplified breast tumors. *, P < 0.05; n = 2–4, average of 3 fields for each section was quantified. C, proteins were isolated from LCC1, LCC9, MCF7, and MCF7-RR human breast cancer cell lines and Western blotting hybridization conducted to measure GRP78 protein expression. Immunohistochemical staining of LCC1 and LCC9 orthotopic tumor sections using a GRP78-specific antibody, n = 4. D, representative tumor sections from tamoxifen (TAM)-treated and control rat mammary tumors stained with GRP78-specific antibody.