Neurotrophins are well-known retrograde signaling molecules that regulate differentiation and cell survival in many central and peripheral neurons. Neurotrophin-4 (NT-4) is the most recently discovered neurotrophic factor in mammals and, functionally, the least well understood. Little is known about the role of NT-4 in supporting the survival of identified classes of sensory neurons. Currently, the only available data are from cell counts of whole sensory ganglia. Mice lacking NT-4 (NT-4/) exhibit a loss of ~50% of the neurons in the nodose-petrosal and geniculate ganglia but no apparent loss of neurons in the dorsal root ganglia.