Among the histological tumor subtypes, the lowest ESR1 expression of malignant tumors was found in solid carcinomas (fresh frozen p = 0.011; FFPE p = 0.003), ductal carcinomas (fresh frozen p = 0.015; FFPE p = 0.0007), and anaplastic carcinomas (fresh frozen p = 0.014), compared to the non-neoplastic mammary tissue (fresh frozen) and lobular hyperplasias of the mammary gland (FFPE), respectively (Fig 1 and S5 Fig). Here, ESR1 is linked to undifferentiated carcinoma.