It has been hypothesized that the capacity to expand in vitro the quantity of hematopoietic stem and progenitor cells available for transplantation would enhance hematopoietic engraftment, reduce the risk of infection, so increasing the number of potentially available alternative donors. Moreover, successful ex vivo expansion of hematopoietic stem cells could be used in many clinical applications, including tumor purging, increasing number of cells available for genetic modification, and generating large quantities of immunologically reactive cells for adoptive immunotherapy.