For example, neurotrophin-3 (NT-3), a critical neurotrophic factor for the survival and maintenance of proprioceptive neurons, serves as a chemoattractant, guiding the proper outgrowth and terminal branching of sensory axons.213, –215 Mice lacking NT-3, its receptor, tyrosine kinases receptor (TrKC) or Runnx3 exhibit severe ataxia and fail to develop muscle spindles as the axons were not able to innervate the target muscles. The gene discussed is NTF3; the disease is Ataxia.