Other spindle cell tumors occurring in the thyroid gland, such as spindle cell atypical thyroid adenoma, spindle epithelial tumor with thymus-like differentiation, poorly differentiated thyroid carcinoma, and anaplastic thyroid carcinoma, may show spindle cell morphology and positive expression of epithelial cell markers, TTF-1 and TG, but they are negative for neuroendocrine markers. This evidence concerns the gene TTF1 and thyroid gland undifferentiated (anaplastic) carcinoma.