The most common breast cancers are ER-positive and/or PR-positive and encompass two main molecular classifications: luminal A, which is HER2-negative and has low expression of Ki67, a marker of proliferation (35.6%), and luminal B, which may be either HER2-positive (13.1%) or -negative (22.5%) but has a high expression of Ki67 and grows faster [6]. Here, ESR1 is linked to breast cancer.