Thrombopoietin (TPO) is a protein produced primarily in the liver that controls how many platelets are made. In people with chronic Idiopathic Thrombocytopenia (ITP) there usually isn't enough TPO (yellow red) reaching the bone marrow when platelet levels get low. This means that there are not enough platelets being produced by bone marrow cells calledmegakaryocytes to make up for the ones that have been destroyed. TPO Receptor Agonist (green) works by stimulating megakaryocytes and their precursors in the bone marrow, leading to proliferation and differentiation of these cells. Normally, when there is a drop in platelet levels, more circulating TPO becomes available to stimulate platelet production. In patients with ITP, there is no compensatory increase in TPO synthesis tostimulate platelet production so the TPO agonist acts to stimulate the protein and promotecontinued activation of the megakaryocytes to produce more platelets.