ERBB3 and cancer: Binding to HER2 strongly inhibits HER2/HER3 downstream signaling, resulting in inhibition of proliferation and induction of apoptosis in HER2-expressing cancer cell lines.9 In breast and gastric HER2-expressing patient-derived xenograft models, MP0274 shows superior antitumor activity compared with trastuzumab and lapatinib, and equivalent efficacy with trastuzumab plus pertuzumab.10 MP0274 does not have an Fc domain and so does not induce antibody-dependent cellular cytotoxicity/complement.