Epiregulin is a member of the epidermal growth factor (EGF) family, and has certain characteristics that are di erent from that of EGF, including mitogenic responses and binding to EGF receptor (EGFR). Epiregulin may also have another cell surface receptor and/or induces di erent receptor heterodimerizations for intracellular signaling. We investigated the binding ability of epiregulin to four ErbB family receptors using four human breast carcinoma cell lines that expressed di erent subsets of receptors. Chemical cross-linking experiments showed that [ 125 I]epiregulin directly bound to each of EGFR and ErbB-4 but not to ErbB-2 and ErbB-3. Furthermore, although epiregulin stimulated tyrosine phosphorylation of all four ErbB receptors, the main intracellular signal was mediated by ErbB-4 and/or EGFR. The pattern of activation of ErbB family receptors was di erent from that of other EGF-related ligands. Our ®ndings indicate that ErbB-4 and EGFR are receptors for epiregulin, and suggest that EGFrelated ligands transduce signals for di erent biological responses by the hierarchical mechanism.