Because DKK1 levels in circulation are increased in ER+ patients with bone metastases36 and Dkk1 is highly expressed in bone in homeostatic conditions43, we evaluated the expression of bone-derived Dkk1 in mice bearing orthotopic breast tumors and found increased expression compared to no tumor controls (Supplementary Fig. 1E, F). Here, DKK1 is linked to breast neoplasm.