To assess whether PFS varied across clinical subgroups, the included studies were subgrouped as: “ET schemes”, “treatment line of advanced disease”, “menopausal status”, “type of CDK4/6 inhibitors”, “age”, “site of metastatic disease”, “histopathological classification”, “prior neoadjuvant or adjuvant CT”, “prior neoadjuvant or adjuvant ET”, “ECOG”, “progesterone receptor status”, “measurable disease”, “disease setting”, “disease-free interval (DFI)” (Fig 4) and “race” (Fig 5). The gene discussed is PGR; the disease is metastatic neoplasm.