Abstract: The invention provides for DNA encoding Fas ligand muteins and chimeras and the proteins encoded thereby. The invention further includes the use of DNA and vectors to produce transformed cells expressing the mutant or chimeric Fas ligand. When the Fas ligand of the invention is a non cleavable form, the cells expressing the Fas ligand are useful in vitro for identifying Fas expressing cells and in vitro or in vivo for reducing populations of Fas expressing cells. Thus, in other embodiments, the present invention is also directed to a method for treating a patient, for example a mammal, for autoimmune disease or transplant rejection by administering a Fas ligand therapeutic agent. The therapeutic agent is a polypeptide, a polynucleotide encoding the polypeptide or a small molecule. The polypeptides include full-length Fas ligand polypeptide, or a biologically active variant, derivative, portion, fusion or peptide thereof.