Signaling by the neurotrophin receptor tyrosine kinase NTRK2 (TRKB) can be activated by binding to neurotrophin-4 (NTF4, also known as NT-4), which functions as a ligand for NTRK2 (Klein et al. 1992, Ip et al. 1993, Ohira et al. 2001). Binding to NTF4 triggers NTRK2 dimerization (Ohira et al. 2001) and trans-autophosphorylation of NTRK2 dimers on conserved tyrosine residues in the cytoplasmic tail of the receptor (Minichiello et al. 1998). Phosphorylated tyrosine residues subsequently serve as docking sites for recruitment of effector proteins that trigger downstream signaling cascades.