As both ErbB3 and ErbB4 have been identified as oestrogen-suppressed and antihormone-induced genes in MCF-7 breast cancer cells [22], we have examined whether upregulation of these receptors is an early response to the pure anti-ER fulvestrant in a panel of four ER-positive breast cancer cell lines, two HER2-overexpressing and two HER2 low-expressing, and if so, what effect ligand activation of these receptors has on the acute growth-inhibitory activity of this antihormone. The gene discussed is ERBB3; the disease is breast cancer.