Peripheral ganglion neurons confer sensory information including touch, pain, temperature, and proprioception. Sensory modality is linked to specific neurotrophin (NTF) requirements. NT-3 supports survival of neurons that differentiate primarily into proprioceptors whereas nerve growth factor and brain-derived neurotrophic factor (BDNF) support subpopulations that transmit nociception and mechanoreception, respectively. We examined sensory neurons of gene-targeted mouse mutants at the NT-4, BDNF, NT-3, and TrkA loci. We show that NT-4 functions early in gangliogenesis, upstream of BDNF. In the absence of NT-4 function, BDNF-dependent, TrkB-expressing neurons fail to appear. The results are consistent with the model that precursor cells intended to become BDNF-dependent mechanoreceptors instead differentiate into NT-3-dependent proprioceptive neurons.