ESR1 and in situ carcinoma: Precursor lesions present a higher expression of oestrogen receptors, which are involved in the transition from normal cells to hyperplastic lesions and then to carcinoma in situ [19,51], strongly suggesting that ER status assessment is critical for the clinical management of premalignant breast disease, being one of the deciding factors when choosing chemoprevention, such as antihormonal drugs [52].