ERBB2 and breast cancer: It is noteworthy that the de novo enrichment of EMT traits, which appears to be a major determinant of primary resistance to trastuzumab in basal/cHER2+ BC cells, and the spontaneous acquisition of EMT traits, which may constitute a major determinant of acquired resistance to trastuzumab in HER2e and luminal (A and B)/cHER2+ BC cells, both converge on a significant decrease in HER2 expression.