The application provides polypeptides comprising or essentially consisting of at least one heavy chain variable domain of a heavy chain antibody (VHH) or functional fragments thereof, wherein said VHH or functional fragment thereof specifically binds to a target protein that is present on and/or specific for a solid tumor or cancer cell, 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 the prevention and/or treatment of cancer.