(2) Undifferentiated carcinoma of thyroid: The tumor cells of undifferentiated thyroid carcinoma are positive for TTF-1, TG, CK19 and negative for calcitonin, whereas the tumor cells of melanin-producing MTC are positive for calcitonin and TTF-1 and negative for TG and CK19. The gene discussed is TTF1; the disease is neoplasm.