ERBB2 and breast cancer: Of note, ERBB4 activation, induced by stimulation with neuregulin 1 (NRG1), has been shown to promote the differentiation and impair the growth of SUM44 breast cancer cells, which belong to the luminal subtype (32) and express very high levels of ERBB4 (Supplementary Figure S2B) and low levels of ERBB2 (10).