Provided herein are polypeptides that are capable of crossing the cellular membrane and entering into the intracellular environment, which polypeptides are suitable for use in prophylactic, therapeutic or diagnostic applications as well as in screening and detection. Nucleic acids encoding such polypeptides; methods for preparing such polypeptides, host cells expressing or capable of expressing such polypeptides, compositions, and in particular pharmaceutical compositions, that comprise such polypeptides, in particular for prophylactic, therapeutic or diagnostic purposes are also provided.