At the latest, 2021 St Gallen consensus meeting, a majority of panellists (62%) agreed with the statement of the International Ki-67 Working Group that in women with ER-positive HER2-negative T1–2 N0–1 breast cancer a low Ki-67 ≤5% would not warrant chemotherapy, whereas a Ki-67 ≥30% would justify chemotherapy. This evidence concerns the gene ESR1 and breast cancer.