ERBB2 and breast cancer: Importantly, the luminal breast cancer subtype expresses high levels of ERBB4 (please consult Figure 1E) and ERBB3 (10) and quite low ERBB2 and EGFR levels (50) [reviewed in (51)]; by consequence, the activation of ERBB4 by ligands in this breast tumor subtype is expected to preferentially activate ERBB4-ERBB4 homodimers or ERBB4-ERBB3 heterodimers.