The present invention relates to amino acid sequences that are capable of binding to serum proteins; to compounds, proteins, polypeptides, fusion proteins or constructs comprising or essentially consisting of such amino acid sequences; to nucleic acids that encode such amino acid sequences, compounds, proteins, polypeptides, fusion proteins or constructs; to compositions, and in particular pharmaceutical compositions, that comprise such amino acid sequences, compounds, proteins, polypeptides, fusion proteins or constructs; and to uses of such amino acid sequences, compounds, proteins, polypeptides, fusion proteins or constructs.