An isolated bi-specific antigen binding construct comprising: a first antigen-binding polypeptide construct which monovalently and specifically binds to one or more amino acid residues in the region from 529 to 625 of HER2, in extracellular domain 4 of HER2 (human epidermal growth factor receptor 2), selected from: i) a polypeptide construct comprising the six CDR sequences of trastuzumab as identified by Chothia; and ii) a polypeptide construct that comprises a first polypeptide comprising the VH amino acid sequence of trastuzumab and a second polypeptide comprising the VL amino acid sequence of trastuzumab; a second antigen-binding polypeptide construct which monovalently and specifically binds to an extracellular domain of HER3 (human epidermal growth factor receptor 3), selected from i) a polypeptide construct comprising the six CDR sequences of H3 as identified by IMGT and; ii) a polypeptide construct that comprises a first polypeptide comprising the VH amino acid sequence of H3 and a second polypeptide comprising the VL amino acid sequence of H3; and a heterodimeric Fc comprising a first Fc polypeptide and a second Fc polypeptide, the first Fc polypeptide linked to the C-terminus of the first antigen-binding polypeptide construct with or without a linker and the second Fc polypeptide linked to the C-terminus of the second antigen-binding polypeptide construct with or without a linker and the heterodimeric Fc comprises a modified CH3 domain comprising the following amino acid modifications, where "A" represents the amino acid modifications to a first CH3 sequence, and "B" represents the amino acid modifications to a second CH3 sequence: A:L351Y_F405A_Y407V, B:T366L_K392M_T394W, A:L351Y_F405A_Y407V, B:T366L_K392L_T394W, A:T350V_L351Y_F405A_Y407V, B:T350V_T366L_K392L_T394W, A:T350V_L351Y_F405A_Y407V, B:T350V_T366L_K392M_T394W, A:T350V_L351Y_S400E_F405A_Y407V, and/or B:T350V_T366L_N390R_K392M_T394W; and wherein the first antigen-binding polypeptide construct is a Fab format and the second antigen-binding polypeptide construct is an scFv format or the first antigen-binding polypeptide construct is an scFv format and the second antigen-binding polypeptide construct is a Fab format, and wherein the isolated bi-specific antigen binding construct displays greater maximum binding (Bmax) in cells expressing HER2 and HER3, compared to trastuzumab. a first antigen-binding polypeptide construct which monovalently and specifically binds to one or more amino acid residues in the region from 529 to 625 of HER2, in extracellular domain 4 of HER2 (human epidermal growth factor receptor 2), selected from: i) a polypeptide construct comprising the six CDR sequences of trastuzumab as identified by Chothia; and ii) a polypeptide construct that comprises a first polypeptide comprising the VH amino acid sequence of trastuzumab and a second polypeptide comprising the VL amino acid sequence of trastuzumab; i) a polypeptide construct comprising the six CDR sequences of trastuzumab as identified by Chothia; and ii) a polypeptide construct that comprises a first polypeptide comprising the VH amino acid sequence of trastuzumab and a second polypeptide comprising the VL amino acid sequence of trastuzumab; a second antigen-binding polypeptide construct which monovalently and specifically binds to an extracellular domain of HER3 (human epidermal growth factor receptor 3), selected from i) a polypeptide construct comprising the six CDR sequences of H3 as identified by IMGT and; ii) a polypeptide construct that comprises a first polypeptide comprising the VH amino acid sequence of H3 and a second polypeptide comprising the VL amino acid sequence of H3; and i) a polypeptide construct comprising the six CDR sequences of H3 as identified by IMGT and; ii) a polypeptide construct that comprises a first polypeptide comprising the VH amino acid sequence of H3 and a second polypeptide comprising the VL amino acid sequence of H3; and a heterodimeric Fc comprising a first Fc polypeptide and a second Fc polypeptide, the first Fc polypeptide linked to the C-terminus of the first antigen-binding polypeptide construct with or without a linker and the second Fc polypeptide linked to the C-terminus of the second antigen-binding polypeptide construct with or without a linker and the heterodimeric Fc comprises a modified CH3 domain comprising the following amino acid modifications, where "A" represents the amino acid modifications to a first CH3 sequence, and "B" represents the amino acid modifications to a second CH3 sequence: A:L351Y_F405A_Y407V, B:T366L_K392M_T394W, A:L351Y_F405A_Y407V, B:T366L_K392L_T394W, A:T350V_L351Y_F405A_Y407V, B:T350V_T366L_K392L_T394W, A:T350V_L351Y_F405A_Y407V, B:T350V_T366L_K392M_T394W, A:T350V_L351Y_S400E_F405A_Y407V, and/or B:T350V_T366L_N390R_K392M_T394W; and wherein the first antigen-binding polypeptide construct is a Fab format and the second antigen-binding polypeptide construct is an scFv format or the first antigen-binding polypeptide construct is an scFv format and the second antigen-binding polypeptide construct is a Fab format, and wherein the isolated bi-specific antigen binding construct displays greater maximum binding (Bmax) in cells expressing HER2 and HER3, compared to trastuzumab.