The present invention establishes the fact that the degradation of SP to SP(1-4) by endogenous NEP in BM stroma can be a mediator of hematopoietic stimulation by stem cell factor (SCF) and induce the production of TGF-.beta. and TNF-.alpha. in BM stroma. The present invention establishes that compositions containing the SP(1-4) polypeptide, or NEP genetic elements, can be used to slow and or stop the rapid growth of stem and progenitor cells thus protecting them from the deleterious effects of cancer therapy. Hence, the polynucleotides and proteins of the present invention may be used to protect stem cells from the toxic effects of chemo- and radio-therapy, in those undergoing, or about to undergo such cancer related treatments. Also provided are compositions containing NEP antisense sequences and antibodies used for the increased proliferation and differentiation of stem and/or progenitor cells in those whom have already undergone chemo- and/or radio-therapy.