The application provides polypeptides comprising or essentially consisting of at least one heavy chain variable domain of a heavy chain antibody (VHH) or a functional fragment thereof, wherein said VHH or a functional fragment thereof specifically binds to a target protein that is present on and/or specific for a solid tumor, e.g. HER2. The application further provides nucleic acids encoding such polypeptides methods for preparing such polypeptides host cells expressing or capable of expressing such polypeptides compositions, and in particular to pharmaceutical compositions, that comprise such polypeptides, nucleic acids and/or host cells. The application further provides such polypeptides, nucleic acids, host cells and/or compositions, for use in methods for detection, imaging, prognosis and diagnosis of cancer as well as for predicting patient response(s) to therapeutics.
