ESR1 and breast cancer: <h4>Purpose</h4>This trial investigated the safety and tolerability of camizestrant with cyclin-dependent kinase 4/6 inhibitors (CDK4/6i) in women with estrogen receptor-positive, HER2- advanced breast cancer.<h4>Patients and methods</h4>SERENA-1 (NCT03616587) is a phase I, multipart, open-label study in women with refractory estrogen receptor-positive, HER2- advanced breast cancer.