ESR1 and breast cancer: BMETs are uniquely increased (2-fold) in metastatic ER+ breast cancers as compared to metastases at other sites, where metastatic prevalence is either the same or reduced as compared to ER− tumors, and osteolysis is a bone- and tumor-specific event (e.g., primarily osteolytic in breast cancer vs. osteosclerotic in prostate cancer) known to be dependent on tumor-derived factors, such as PTHrP[8–11].