Estrogen Receptor–Positive, Progesterone Receptor–Negative Breast Cancer: Association With Growth Factor Receptor Expression and Tamoxifen Resistance

ER+/PR- tumors express higher levels of HER-1 and HER-2 and display more aggressive features than ER+/PR+ tumors. As in laboratory models, lack of PR expression in ER+ tumors may be a surrogate marker of aberrant growth factor signaling that could contribute to the tamoxifen resistance observed in these tumors.