An isolated polypeptide is provided. The isolated polypeptide comprising an antigen recognition domain specifically binding human HER-3 with a KDvalue of 10 nM or lower, wherein the polypeptide inhibits neuregulin (NRG) binding to the human HER3 and NRG-induced cancer cell migration and proliferation. Additionally clones NG83 and NG140 are provided which bind human HER-3 with a KDvalue of 10 nM or lower.