The invention provides an isolated monoclonal antibody or antigen binding portion thereof that binds to human ErbB3 and comprises: (a) a heavy chain variable region comprising the amino acid sequence of SEQ ID NO: 1 and a light chain variable region comprising the amino acid sequence of SEQ ID NO: 2; or (b) a heavy chain variable region comprising an amino acid sequence which is at least 80% identical to the amino acid sequence of SEQ ID NO: 1 and a light chain variable region comprising an amino acid sequence which is at least 80% identical to the amino acid sequence of SEQ ID NO: 2.
