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.
