NTRK1 and neoplasm: To validate these results and correlate CD271 expression with the tumor stage, we evaluated CD271 protein expression in human biopsies of AK or in situ, WD or MD/PD cSCC, together with differentiation (Keratin 1 – KRT1) [32] and proliferative and aggressiveness markers (Cyclin D1, Keratin 13 – KRT13, and TrkA) [30, 33, 34] (Fig. 1a).