Abstract: The present invention provides a method of treating a disease or condition associated with signalling via the TPO receptor. The method comprises the step of administering an effective amount of a peptidyl TPO receptor antagonist to a subject in need thereof, wherein the peptidyl TPOR receptor antagonist is a cyclic or linear peptidyl compound comprising the following structural formula (I): Xbb-Xaa-Xcc wherein Xbb represents a residue of an amino acid selected from arginine (R) and lysine (K); Xaa represents a residue of an amino acid selected from glutamine (Q), asparagine (N), aspartic acid (D), and glutamic acid (E); Xcc represents a residue of an amino acid selected from tryptophan (W), phenylalanine (F), tyrosine (Y), and histidine (H); or a salt thereof.