Ex vivo expansion of hematopoietic stem and progenitor cells has been a major goal for experimental hematologists and stem cell biologists during the last two decades. The clinical implications of such a procedure are obvious, considering the increasing interest in cell therapy protocols. This is particularly true in the setting of cord blood transplants, in which increased numbers of such primitive cells are needed. The study analyzed in this article indicates that by stimulating the Notch signal transduction pathway in primitive cord blood cells it is possible to significantly increase the numbers of both hematopoietic stem and progenitor cells. Furthermore, infusion of such expanded cells in patients receiving a cord blood transplant results in a significant reduction in the time to myeloid engraftment. The relevance of this study is twofold--on the one hand, it shows that the Notch pathway is involved in the expansion capacity of primitive hematopoietic cells in culture, and on the other hand, it indicates that ex vivo-expanded stem/progenitor cells can have a role in hematopoietic transplantation settings.