A method of maintaining undifferentiated biological cells in vitro comprising culturing the cells in the presence of NM23 protein, without any cytokines other then cytokines IL3 and /or Stem cell factor (SCF), wherein the NM23 acts to inhibit differentiation and maturation of said cultured cells, and wherein the cultured cells are stem cells. The invention also relates to a cell culture medium comprising NM23 proteins that inhibit maturation and differentiation of the cultured stem cells.