NGF and skin squamous cell carcinoma: The mechanism of PNI is not well defined; thus, it has been researched that neural cell adhesion molecules, such as the expression of TrK-A (tropomyosin receptor kinase A), a high-affinity receptor for the nerve growth factor which participates in the maintenance of the central and peripheral nervous system, is almost double in head and neck CSCC compared to non-head and neck areas, but it lacks statistical correlation between TrK-A expression and PNI in CSCC [9,13,17].