The present invention relates to a method of expanding a population of CD34+ hematopoietic stem cells ex-vivo, while at the same time, inhibiting differentiation of the stem cells ex-vivo, the method comprising: (a) culturing said CD34+ hematopoietic stem cells ex-vivo under conditions allowing for cell proliferation, said conditions which comprise providing nutrients, serum and a combination of cytokines including stem cell factor, thrombopoietin, FLt3 ligand, IL-6 and IL-3, and (b) in the same culture medium providing nicotinamide in an amount between 1.0 mM to 10 mM, wherein said cells are cultured for a culture period resulting in expanding the population of CD34+ hematopoietic stem cells while inhibiting differentiation of said CD34+ hematopoietic stem cells ex-vivo to produce an expanded CD34+ hematopoietic stem cell population with an increased proportion of CD34+/Lin- and CD34+/CD38- cells in the expanded culture as compared to CD34+ cells cultured in the presence of cytokines and nutrients without exogenously added nicotinamide.
