Neurotrophin–3 (NT–3) is a member of the neurotrophin family, which includes nerve growth factor (NGF), brain–derived neurotrophic factor (BDNF) and neurotrophin–4/5 (NT–4/5). These factors are crucial for development of the peripheral nervous system1, but not the central nervous system (CNS), except that NT–3 and BDNF have been implicated in the postnatal development of the cerebellum2,3. Here we created a conditional NT–3–deficient mutant, which showed abnormal cerebellar morphology.