ERBB2 and breast cancer: As the majority of patients analyzed during the observation period between 2010 and 2020 did not receive a CDK 4/6 inhibitor, the mean 5-year OS rate of 41.8% among primary metastatic HR+/HER2− breast cancer patients is consistent with data published in the era before CDK 4/6 inhibitors were introduced [28,32,40,41].