A drug that regulates platelet production in the bone marrow. TPO is a genetic copy of a naturally occurring hormone. Helpful in lessening the severity and duration of thrombocytopenia (low platelet levels) that may occur in cancer therapy treatments. Synonyms for TPO are c-Mpl ligand, mpl ligand, megapoietin and megakaryocyte growth and development factor (MGDF or PEG-rHuMGDF) (a truncated version of thrombopoietin). TPO is currently in clinical trials.