In the present study, we have examined the acute inductive capacity of the pure anti-ER fulvestrant on ErbB3 and ErbB4 receptor expression in a panel of four ER-positive breast cancer cell lines, two ErbB2-negative (MCF-7 and T47D) and two ErbB2-positive (BT474 and MDAMB361), and assessed the effect of ligand activation of these receptors on antihormone response. This evidence concerns the gene ERBB3 and breast carcinoma.