BDNF and cancer: Molecules secreted by cancer-activated Schwann cells that have been implicated in PNI include neural cell adhesion molecule 1 (NCAM1), L1 cell adhesion molecule (L1CAM), transforming growth factor β (TGFβ), C-C motif chemokine ligand 2 (CCL2), matrix metalloproteinase (MMP) 2, MMP9 (MMP9), nerve growth factor (NGF), and brain-derived neurotrophic factor (BDNF) [38,40,41,42,43,44,45].