HER2 binding polypeptides comprising the amino acid sequence EX 1 RNAYWEIA LLPNLTNQQK RAFIRKLYDD PSQSSELLX 2 E AKKLNDSQ wherein X 1 in position 2 is M, I or L, and X 2 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. Nucleic acids encoding the polypeptides, expression vectors comprising the nucleic acids and host cells comprising the expression vectors are also disclosed.
