TTF1 and squamous cell carcinoma: TRU carcinomas are TTF-1-positive, highly differentiated tumors that arise from type 2 alveolar epithelial cells and Clara cells, whereas non-TRU carcinomas are TTF-1-negative tumors that arise from dysplastic mucosa columnar cells located in the central part of non-TRU carcinomas, which often coexist with squamous cell carcinomas (sqCC).