In this context, immunohistochemistry (IHC) can support a differential diagnosis between TM and primary TC, which are mostly TG, TTF-1 and PAX8 positive [60], whereas these markers are negative in secondary thyroid malignancy; ER, PR, HER2, GATA3 and GCDFP15 are peculiar of BC-derived tissues [13,61]. The gene discussed is TTF1; the disease is breast cancer.