BDNF and neoplasm: There is a variety of factors involved in the regulation of tumor–nerve interactions; among others, these are neurotransmitters and their receptors (NE/ADR and Ach/AChR), neurotrophins and their receptors (NGF/TRK/p75NTR, NT-3/TRKC, NT-4/5, BDNF/TRKB, and GDNF/RET/GFRα), chemokines and their receptors (CX3CR1/CX3CL1, CCR2/CCL2, and CXCR4/CXCL12), metalloproteinases or cell-surface molecules and their receptors (L1-CAM, NCAM, semaphoring 3A/plexins A1-A4, neuropilin-1, and semaphorin 4F) [57,59].