The invention provides an antigen-binding protein that specifically binds to a conformational epitope formed by domain III & IV of human epidermal growth factor receptor 3 (HER3) and antigen-binding proteins which compete therewith for binding, as well as fusion protein or conjugate comprising these. The invention also provides nucleic acid molecule comprising a sequence encoding said antigen binding proteins, vectors comprising the nucleic acid, and cells and pharmaceuticals comprising the antigen binding protein, the fusion protein, the nucleic acid, or the vector. The invention also provides the antigen binding protein, the fusion protein or conjugate, the nucleic acid, the vector, the cell, or the pharmaceutical for use as a medicament. The invention further provides a method of inhibiting tumor growth or treating cancer, comprising administering a therapeutically effective amount of the antigen binding protein, the fusion protein or conjugate, the nucleic acid, the vector, the cell, or the pharmaceutical.