ESR1 and medullary thyroid gland carcinoma: Medullary carcinoma has long garnered interest due to its morphological and immunophenotypical association with two of the most controversial entities in breast cancer: BRCA1-linked tumors, which often exhibit a much higher rate of MBC and are very frequently estrogen receptor (ER)-negative, and basal-like cancers, which express at least one basal marker[3].