TTF1 and Alzheimer disease: In particular, PDSCC and non-lepidic AD are difficult to be distinguished in the clinical setting when relying on protein markers such as napsin A [24, 25] and TTF-1 [24, 25] (AD markers), or p40 [26, 27], DSG3 [24, 28], CK5 [24, 25] and CK6 [25] (SCC markers).