They are a source of growth factors (GFs), such as glial cell line-derived neurotrophic factor (GDNF), basic fibroblast growth factor (bFGF), brain-derived nerve growth factor (BDNF), nerve growth factor (NGF), and adhesion molecules [29], that allow them to promote axonal regeneration [30,31,32], remyelination [33,34,35] and functional recovery in spinal cord injury (SCI) [36]. The gene discussed is GDNF; the disease is spinal cord injury.