ESR1 and breast cancer: Bone is the most common site of metastasis in patients with breast cancer(1) and occurs in approximately 60% of metastatic estrogen receptor‐positive (ER+) patients and 40% of metastatic triple negative breast cancer (TNBC) patients.(2, 3) Despite the high prevalence of bone metastasis, relatively few therapeutic options to effectively target bone metastases exist, and thus metastatic breast cancer remains incurable.