Glial cell line-derived neurotrophic factor (GDNF) and the related neurotrophic factor neurturin (NTN) are potent survival factors for central and peripheral neurons. GDNF is a glycosylated, disulfide-bonded homodimer that is distantly related to the TGF Beta superfamily of growth factors. Three receptors for these factors, GFR Alpha-1 (also designated GDNFR-Alpha, RETL1 or TrnR-1), GFR Alpha-2 (also designated GDNFR-Beta, RETL2, NTNR-Alpha or TrnR-2) and GFR Alpha-3 have been identified. Th