ERBB3 and breast cancer: These observations are consistent with other regimens targeting multiple HER receptors, and likely indicative of HER3-mediated toxicity [e.g., pan-HER TKI afatinib vs. cetuximab in R/M SCCHN (20); addition of pertuzumab (inhibitor of the dimerization of HER2/HER3) to trastuzumab and chemotherapy in 1L HER2 + breast cancer (21)].