Abstract: HER2 binding polypeptides comprising the amino acid sequence EX1 RNAYWEIA LLPNLTNQQK RAFIRKLYDD PSQSSELLX2E AKKLNDSQ wherein X1 in position 2 is M, I or L, and X2 in position 39 is S or C (SEQ ID NO:1) are disclosed. Moreover, such peptides comprising a chelating environment are disclosed. Also radiolabeled polypeptides formed by the peptides comprising a chelating environment and radionuclides are disclosed. Furthermore, methods of in vivo imaging of the body of a mammalian subject having or suspected of having a cancer characterized by overexpression of HER2 comprising administration of such a radiolabeled polypeptide followed by obtainment of an image of the body using a medical imaging instrument and also methods of treating such cancer are disclosed. Furthermore, the use of such a radiolabeled polypeptide in diagnosis and treatment of cancer characterized by overexpression of HER2.