The invention relates to the stem cells, embryonic stem cells in particular. It has been found that, when these stem cells are entrapped such that their proliferation is inhibited, they produce material which inhibits the proliferation of other, non-entrapped cells, including stem cells and neoplastic and/or hyperproliferative, but otherwise normal cells. It has also been found that entrapped cancer cells will produce material which inhibits the proliferation of stem cells. Further, it has been found that the entrapment of the stem cells inhibits their differentiation and thus the entrapment process can serve as a long-term storage device for maintaining the undifferentiated state of at least a portion of the entrapped cells.