Binding specificities of ErbB receptor family ligands. EGF (epidermal growth factor), AREG (amphiregulin) and TGFA (transforming growth factor-α) bind EGFR (ErbB1) only. BTC (Betacellulin), HBEGF (heparin-binding EGF-like growth factor) and EREG (Epiregulin) bind EGFR and ErbB4. NRG1 and NRG2 bind ErbB3 and ErbB4. NRG3 and NRG4 are specific for ErbB4. Receptors form homo- and heterodimers after ligand-binding (not shown). As indicated (X), ErbB2 has no ligand-binding capacity and ErbB3 has no active kinase domain. Only NRG1-4 and ERBB1-4 genes were investigated in this study. (Modified from ).