Patel et al. (2003) also present observations from islet2DTA mice, which lack a significant portion of the motor neurons in the ventral cord. In these mice PV immunostaining shows axons in the ventral horns. Patel et al. argue that since motor neurons are absent in these mice, NT-3 secreted by them could not be a signal for proprioceptive axons to enter the lateral motor columns. However, there is no evidence showing that NT-3 mRNA or protein expressed in the ventral spinal cord is exclusively from motor neurons, and there are no available data indicating that in islet2DTA mice, NT-3 expression in the ventral spinal cord is abolished (Yang et al. 2001; Pun et al. 2002). Studies in embryonic mice reported NT-3 mRNA in the ventral horns of the spinal cord, but it is not definitive that both mRNA and protein are expressed solely by motor neurons. In the adult spinal cord, while motor neurons express high levels of NT-3, other cells, including glia, also express it (Zhou and Rush 1994; Dreyfus et al. 1999; Buck et al. 2000). Our present results, along with those from transgenic mice with NT-3 over-expression in ectopic regions of the spinal cord (Ringstedt et al. 1997), argue for a role of NT-3 in chemoattractant axon guidance of proprioceptive axons in the spinal cord.