TTF1 and small cell carcinoma: In particular, reports indicate that as many as 81% to 97% of SCLCs express TTF-1, whereas it is never or rarely expressed in squamous cell carcinoma of the lung.8–10,12–14 Regarding small cell carcinoma in general, TTF-1 is expressed not only in SCLC but also in small cell carcinoma of other organs, although the frequency of expression varies.12,15 Therefore, concerning SCLC, TTF-1 is not necessarily a marker of the lung origin.