POLARIS is a prospective, real-world study of palbociclib in patients with hormone receptor–positive (HR+)/human epidermal growth factor receptor 2–negative (HER2–) advanced breast cancer (ABC) in the United States and Canada....Patients in the biomarker analysis group provided consent for serial blood sample collection, received ≥1 dose of initial palbociclib combination treatment...Patients with mutated ESR1 and PIK3CA or CCND1 and FGFR1 amp at baseline had shorter rwPFS than patients with wild-type genes .