TTF1 and small cell lung carcinoma: Immunohistochemical analysis revealed high positivity rates for CK, Syn, CD56, and TTF-1 in SCLC, corroborating previous reports.[20,21] The Ki-67 index, indicative of cell proliferation capacity, has been widely recognized for differentiating SCLC from carcinoids, although it does not distinguish between typical and atypical carcinoids.[22,23]