Over the last decade the use of umbilical cord blood as a source of hematopoietic stem cells (HSC) has increased due to its quick availability and because it allows the usage of mismatched donors, thus reducing the risk of graft versus host disease. However, one major limitation of umbilical cord blood as an HSC therapy is the low cell dose available for transplantation. Hematopoietic stem cells are defined by their capacity to self-renew and reconstitute the entire blood system of a transplanted recipient. In mouse, the use of cell surface markers has allowed an accurate HSC isolation by fluorescence activated cell sorting (FACS). However, human HSC population is still unestablished. It has been described that the combination of the well known Lin- LSK mouse HSC and the SLAM family markers can refine cell hierarchies within mouse HSC population. We have used CD150 and CD135 markers together with the classic CD34+CD38- population, to elucidate if these markers would also define a high enriched population of HSC with a high repopulating potential in humans. We have isolated the CD34+CD38-CD150+CD135- cord blood population and tested its hematopoietic stem cell potential. In vitro and in vivo assays have demonstrated that this population contains short and long-term HSC that can be differentiated into all blood lineages. We have demonstrated that these cells self-renew and reconstitute the hematopoietic system of a transplanted mouse. Moreover, we have observed that most of these cells remain in a quiescent state and they can be activated by culturing them on stromal cells with cytokines. One approach to overcome the obstacle of low cell dose observed in umbilical cord blood transplantation has been the attempt to expand HSC ex vivo. We have cultured hematopoietic stem cells on stromal cells that induce Notch pathway activation. We have also studied the effects of the addition of valproic acid, an epigenetic modifier, on HSC expansion. Hematopoietic stem cell culture on stromal cells that induce Notch pathway activation promote HSC proliferation thus reducing differentiation. Valproic acid treatment has similar effects to stromal culture and Notch activation on HSC. Moreover, VPA induces Hes1 expression by a Notch dependent or independent mechanism. Both studies conclude that hematopoietic stem cell quiescence can be reverted by environmental external signals. Thus activating and inducing proliferation on hematopoietic progenitors.