Bdnf-/- and Nt f5-/- mice lose 50% of geniculate/petrosal and nodose neurons. Mice lacking both BDNF and NT4/5 lose almost all of the neurons in these ganglia. At least two different scenarios could explain these findings. Two separate subpopulations could exist. One that is BDNF-dependent and one is that is NT4/5-dependent (A). In this case, BDNF dependent neurons would be lost in Bdnf-/- mice and NT4/5-dependent neurons are lost in Ntf5-/- mice. Alternatively, one subpopulation of geniculate/petrosal/nodose neurons may be dependent on both BDNF and NT4/5 (C) such that removal of either neurotrophin would result in death. These two possibilities are not mutually exclusive. That is, all four types of neuron dependencies could be present in the same ganglion (B).