Required for axon extension during neural development. Homomultimer. Interacts with NTRK1/TrkA. At 11.5 dpc, expressed in spinal nerves, their roots and the ventral spinal cord. At 12.5 dpc, detected in the ventral spinal cord, dorsal root ganglia (DRG), dorsal and ventral roots and sympathetic chain ganglia. At 12.5 dpc, expressed in almost all motor neurons which also express RET and in almost all DRG sensory neurons which also express NTRK1. At 18.5 dpc, expressed only in a subset of NTRK1-expressing neurons but still expressed in nearly all RET-expressing neurons. Sensory and motor neuron axonal projection defects.