Most studies evaluating TTF-1 by IHC have focused on pulmonary and thyroidal tumors and identified TTF-1 positivity in 42–100% of pulmonary adenocarcinomas [8–11], 15–93% of pulmonary small cell carcinomas [12–15], 0–50% of squamous cell carcinomas of the lung [12, 16–18], and 0–42% of malignant mesotheliomas [19–22] as well as in 88–100% of follicular [23, 24], 66–100% of papillary [23, 24], 50–100% of medullary [25, 26], and 5–100% of anaplastic cancers of the thyroid [27, 28]. This evidence concerns the gene TTF1 and thyroid gland undifferentiated (anaplastic) carcinoma.