The disclosure provides a HER3 binding polypeptide, comprising a HER3 binding motif, BM, which motif consists of the amino acid sequence selected from i) EX2X3X4A X6X7EIW X11LPNL X16X17X18QX20 X21AFIX25 X26LX28D, and ii) an amino acid sequence which has at least 90% identity to the sequence defined in i), wherein the polypeptide binds to the extra-cellular domain of HER3. Also provided is a bispecific ligand having binding affinity for HER3 and for HER2, or for HER3 and for EGFR, and comprising a HER3. binding polypeptide as defined herein and a HER2 binding polypeptide or a EGFR binding polypeptide.