BDNF and neoplasm: Broadly, the neuroactive substances released during tumor–nerve interactions can be categorized into three primary groups: (1) neurotropic factors such as NGF, BDNF, and glial cell line-derived neurotrophic factor and others; (2) axon guidance molecules such as C-C motif chemokine ligand 2 (CCL2), C-X3-C motif chemokine ligand 1 (CX3CL1), Ephrin type-A receptor 2 (EphA2), and Slit; and (3) neurotransmitters including ACh, glutamate, glycine, adrenaline, noradrenaline, and dopamine [97].