Glial cell line-derived neurotrophic factor (GDNF) is a neurotrophic factor and a member of the tumor growth factor (TGF)-beta superfamily. The GDNF family of growth factors also includes neurturin, persephin, and artemin, which have seven conserved cysteine residues called cysteine-knots (Treanor et al.). GDNF family ligands signal through binding to specific GDNF-family receptor-α (GFRα) co-receptors and activate the RET receptor tyrosine kinase (Durbec et al.). Four different forms of GFRα co-receptors have been characterized (GFRα 1-4); GDNF binds specifically to GFRα1 prior to forming a complex with RET (Airaksinen and Saarma). GDNF is known to promote survival and morphological differentiation of midbrain dopaminergic neurons in both in vivo and in vitro studies and increases their high-affinity dopamine uptake (Granholm et al.; Lin et al.). GDNF has also been shown to have restorative effects on dying dopaminergic neurons in response to degenerative toxins (Aoi et al.). GDNF, together with Human Recombinant BDNF (brain-derived neurotrophic factor), BrainPhys™ Neuronal Medium, and other supplements, can be used to differentiate human pluripotent stem cell (hPSC)-derived neural progenitor cells into neurons (Bardy et al.).