Seventy-three percent of retrogradely labeled gastrocnemius muscle afferents were reported to be expressing TrkC RNA in the adult rat DRG, although some of these (about 10%) may represent cutaneous innervation (McMahon et al. 1994). Presently, it is not clear whether TrkC protein is also made by cutaneous afferents and if so at what stage in development this receptor is expressed by cutaneous axons. Based on the available evidence showing that all proprioceptive neurons are eliminated in NT-3 or TrkC null mice, we think that TrkC staining in the tibial nerve mostly represents proprioceptive axons in the vicinity of their peripheral target. Altogether, our findings suggest a role for NT-3 in initiation of muscle innervation and spindle differentiation by the proprioceptive axons.