The present invention relates to amino acid sequences that are directed against (as defined herein) HER3, as well as to compounds or constructs, and in particular proteins and polypeptides, that comprise or essentially consist of one or more such amino acid sequences (also referred to herein as "amino acid sequences of the invention", "compounds of the invention", and "polypeptides of the invention", respectively). The invention also relates to nucleic acids encoding such amino acid sequences and polypeptides (also referred to herein as "nucleic acids of the invention" or "nucleotide sequences of the invention")； to methods for preparing such amino acid sequences and polypeptides； to host cells expressing or capable of expressing such amino acid sequences or polypeptides； to compositions, and in particular to pharmaceutical compositions, that comprise such amino acid sequences, polypeptides, nucleic acids and/or host cells； and to uses of such amino acid sequences or polypeptides, nucleic acids, host cells and/or compositions, in particular for prophylactic, therapeutic or diagnostic purposes, such as the prophylactic, therapeutic or diagnostic purposes mentioned herein.
