Factors underlying geographic atrophy are unknown; however, from studies on the viability and rescue of photoreceptors, it appears that factors produced by RPE cells, including basic fibroblast growth factor (bFGF), brain-derived neurotrophic factor (BDNF), and ciliary neurotrophic factor (CNTF), are responsible for normal functioning of the retina and photoreceptors' viability [100–102]. The gene discussed is BDNF; the disease is atrophic macular degeneration.