ESR1 and breast cancer: The purpose of this ongoing open-label, multicenter, phase 1 clinical study was to evaluate the safety, tolerability and clinical activity of PF-07248144 as monotherapy or as combined therapy in patients with solid tumors, with a focus on locally advanced or metastatic ER+HER2− breast cancer, whose disease progressed after CDK4/6 inhibitors and endocrine therapies.