Moreover, mRNA levels of NGF and TrkA were higher in tumors with perineural invasion than in those without perineural invasion, and perineural invasion was more likely to occur in tumors with high expression of NGF and TrkA, suggesting that NGF released by cancer cells does not act on itself through paracrine or autocrine actions, but rather acts on TrkA presented at the perineurium to bring nerves into contact with cancer cells, which in turn leads to TrkA-mediated perineural invasion [23]. This evidence concerns the gene NTRK1 and cancer.