In AD patients, nerve fibers show positivity for substance P are elevated as compared to controls.120 Further, another study showed upregulation of TAC1 (the gene encoding SP) in itchy skin, that positively correlated with itch intensity in AD patients.121 Also, the number of mast cells and production of PAMP12 by keratinocytes was enhanced in patients with ACD (n=5) compared to controls (n=5).49 Urticaria, another common and heterogeneous skin condition characterized by itching, hives, and inflammation, is also thought to be driven by aberrant mast cell activity. This evidence concerns the gene TAC1 and urticaria.