We suggest that limited expression of neurotrophins in the immature target leads to retention of primary neurons and their afferent fibers in newborn and early postnatal Brn3c null mice. In agreement with this suggestion, our in situ data show a severe reduction in expression of BDNF (Figs. 8, 9). Despite this reduction, some, apparently biologically significant low levels of neurotrophins are apparently expressed in all sensory epithelia in embryonic Brn3c null mutants. It appears that even these low levels of neurotrophins in the vestibular sensory epithelia can sustain normal fiber outgrowth and a limited maintenance until P0. The comparatively high level of NT-3 expression in Brn3c null mutant cochlea, which has been shown to be the neurotrophin most prominently supporting spiral sensory neurons [6,9,14], is in agreement with the normal development of cochlear innervation in newborn Brn3c null mutants.