MPL and myeloproliferative disorder: As will be discussed below, the role of the thrombopoietin receptor in myeloproliferative neoplasms has driven the search for receptor antagonists and the need to expand hematopoietic stem cells in vitro for a myriad of ex vivo cellular applications, only two of which include expansion for transplantation and manipulation to correct genetic mutations (e.g., sickle hemoglobin) and are but two practical applications of the approach illustrated in the work of Tsutsumi and colleagues [30].