ESR1 and thyroid gland carcinoma: Some researchers have suggested that patients with secondary thyroid cancer have a higher rate of both ER and PR positivity than patients with breast cancer only25,26.ER has two isoforms, Erα and Erβ, and overexpression of ERα in thyroid cancer tissues and lack of expression of ERβ in peripheral tissues was reported in 201127 , and under-expression or deletion of ER can be considered a hallmark of thyroid cancer28.