ESR1 and breast cancer: To address this, we investigated the association of Ki67 gene (MKI67) expression with biological features and treatment response in estrogen receptor-positive (ER+) breast cancer (BC), the most common subtype.<h4>Patients and methods</h4>We analyzed 5036 patients with ER+/HER2- BC across 11 independent cohorts with tumor transcriptomes and clinical data.