Gene knockout experiments were conducted to identify the neuronal populations in both the PNS and CNS that were affected by the loss of different neurotrophins during development and the extent to which these populations were affected. These knockout experiments resulted in the loss of several neuron populations including the retina, cholinergic brainstem and the spinal cord. It was found that NGF-knockout mice had losses of a majority of their dorsal root ganglia (DRG), trigeminal ganglia and superior cervical ganglia. The of these mice was poor. The BDNF-knockout mice had losses of a majority of their vestibular ganglia and moderate losses of their DRG, trigeminal ganglia, nodose petrosal ganglia and cochlear ganglia. In addition they also had minor losses of their facial motoneurons located in the CNS. The viability of these mice was moderate. The NT-4-knockout mice had moderate losses of their nodose petrosal ganglia and minor losses of their DRG, trigeminal ganglia and vestibular ganglia. The NT-4-knockout mice also had minor losses of facial motoneurons. These mice were very viable. The NT-3 knockout mice had losses of a majority of their DRG, trigeminal ganglia, cochlear ganglia and superior cervival ganglia and moderate losses of nodose petrosal ganglia and vestibular ganglia. In addition the NT-3-knockout mice had moderate losses of spinal moroneurons. These mice had very poor viability. These results show that the absence of different neurotrophins result in losses of different neuron populations (mainly in the PNS). Furthermore, the absence of the neurotrophin survival signal leads to apoptosis.