ERBB2 and breast cancer: We then tested the sensitivity of cell lines grown as mammospheres to Lapatinib, an ATP-competitive reversible small-molecule inhibitor of the ErbB2 and EGFR tyrosine kinases currently used in clinics as therapy for Her2-overexpressing metastatic breast cancers resistant to Trastuzumab-based regimens.23 As shown in Figure 1d, cells are resistant to treatment consistently with low expression levels of the receptors.