Abstract: A novel mammalian protein system is disclosed. This system comprises a secreted protein factor and its related membrane-bound splice variant. These novel proteins have no homology to any known protein or class or protein, yet are ubiquitously expressed in nearly all tissue types. Also disclosed are novel nucleic acids that encode the novel polypeptides of the invention. The protein system was discovered in purified populations of murine hematopoieitc stem cells. The cDNAs of the invention were cloned from a murine hematopoietic lineage negative (Lin) library.