ERBB3 and cancer: Second, since heregulin‐expressing cancer cells were sensitive to several anti‐HER3 antibodies including patritumab, we examined whether this agent could overcome heregulin‐mediated resistance.12, 15, 16 The combination of patritumab and trastuzumab inhibited cell viability compared to that with trastuzumab alone; however, the viability curve did not reach the IC50, indicating that this combination was still not sufficient to inhibit heregulin‐expressing cell growth (Figure 2B).