By labelling squamous-like cells only, even in PD carcinomas, involucrin immunoreaction determines the epithelial origin of the neoplasia, allowing the distinction from other carcinomas, namely basal cell tumours or other proliferative lesions formed by basaloid keratinocytes, which do not express detectable immunoreactivity for involucrin, except in focal areas of eventual squamoid differentiation, according to previous statements [24]. The gene discussed is IVL; the disease is basal cell neoplasm.