LIF, GDNF, and GDF15 are potent survival factors for motor neurons [53,54,55,56,57], AGRIN (encoded by Agrn) is secreted by motor neurons at neuromuscular junctions [58], and RNA metabolism is altered in SMA and ALS patients associated with Fus or Tardbp mutations [22]. Here, GDF15 is linked to proximal spinal muscular atrophy.