The POETIC-A (Peri-Operative Endocrine Therapy for Individualizing Care with Abemaciclib) is a multicentre phase 3 clinical trial enrolling post-menopausal women with operable invasive ER+/HER2− BC, with high 5-year risk of tumor relapse (characterized by high baseline levels of Ki67 ≥ 20%, or tumor size > 5cm, grade 3 tumor, and vascular invasion) after 2 weeks of treatment with aromatase inhibitor (AI) therapy before surgery. This evidence concerns the gene ESR1 and breast cancer.