Neither mPFS nor mOS (from chemotherapy initiation) was related to the following factors: patient age, tumor grade, ER (%), PR (%), HER-2 status, Ki67 status, presence of visceral metastases, previous application of chemotherapy in any setting, time from diagnosis to CDK4/6i therapy end, time from diagnosis of metastatic disease to CDK4/6i therapy end, length of CDK4/6i therapy, line of palliative therapy in which CDK4/6i was administered (first line, second line), or administration of any type of chemotherapy before CDK4/6i or type of CDK4/6i was used. Here, ESR1 is linked to metastatic neoplasm.