Abstract: The present invention relates to a synthetic variable region of an immunoglobin construct which contains in at least one of its CDRs a sequence of thrombopoietin, <i>e.g.</i>, IEGPTLRQWLAARA or its derivatives. This construct can efficiently bind and activate a thrombopoientin receptor (MPL) leading to stimulation of proliferation, growth or differentiation or modulation of apoptosis of hematopoietic cells, especially platelet progenitor cells. The invention further relates to the use of the synthebody to treat hematopoietic or immune disorders, and particularly thrombocytopenia resulting from chemotherapy, radiation therapy, or bone marrow transfusions.