Members of the glial cell line-derived neurotrophic factor (GDNF) family, including GDNF and neurturin (NTN) (1), play key roles in the control of vertebrate neuron survival and differentiation. Physiological responses to NTN require the presence of a novel glycosylphosphadidylinositol linked protein NTNRa, which is a cell surface receptor for NTN (2,3). The cDNAs encoding NTNRa from human, rat, chicken, and mouse have been cloned recently (2-7). NTNRa was also termed GDNFRb, Ret ligand 2 (RETL2) or TGF-b-related neurotrophic factor receptor 2 (TrnR2) (4-7) and nominated as GFRa-2 recently (8). GFRa-2 binds NTN and mediates activation of RET receptor tyrosine kinase by both NTN and GDNF. Thus, NTN, GFRa-2, and the Ret PTK form a complex to transduce NTN signal and to mediate NTN function.