The present invention relates to human, rat and mouse stem cell-derived neuron survival factor polypeptides (SDNSF), a process for producing them, cDNA encoding SDNSF, a vector comprising the cDNA, host cells transformed by the vector, an antibody against SDNSF, pharmaceutical compositions containing SDNSF or the antibody, a method of assaying SDNSF, a reagent for assaying SDNSF, and a screening method using SDNSF. The polypeptides are effective in the survival of nerve cells and neuronal stem cells, therefore, efficacious in treating injury to the central nerve system and cancer.