Abstract: Labelled polypeptide constituted by: (i) a peptide fragment of 38 to 50 amino acids including a peptide of sequence SEQ ID NO: 1, and (ii) a label selected from the group constituted by radioactive groups, non-protein fluorophors, protein fluorophors, magnetic particles or a paramagnetic spin label, in order that when the label is a fluorophor protein, the labelled polypeptide further contains a peptide spacer of 3 to 15 amino acids, located between the peptide fragment and the label and use thereof as a diagnostic tool.