TERT and thyroid gland undifferentiated (anaplastic) carcinoma: Papillary, follicular, oncocytic, medullary, and anaplastic thyroid carcinomas harbor distinct early driver mutations, such as BRAFV600E, RAS, and fusion events (RET, NTRK, and ALK), that cooperate with secondary alterations (TERT promoter, TP53, PIK3CA, and CDKN2A/B loss) to drive dedifferentiation, metastasis, and therapeutic resistance.