A human stem cell antigen 2 polypeptide and DNA (RNA) encoding such polypeptide and a procedure for producing such polypeptide by recombinant techniques is disclosed. Also disclosed are methods for utilizing such polypeptide for stimulating thymocyte maturation and differentiation, protecting neuronal cells, preventing rejection during organ transplantation, treating PNH, Alzheimer's Disease and inflammation is disclosed. Diagnostic assays for identifying mutations in nucleic acid sequence encoding a polypeptide of the present invention and for detecting altered levels of the polypeptide of the present invention for detecting diseases, for example, cancer, are also disclosed.