Bone is the most common metastasis site in patients with breast cancer, and approximately up to 80% of patients with advanced breast cancer (BC) develop bone metastasis (BM), which leads to a worse prognosis with a median survival of 16 months and skeletal-related events (SREs), including severe bone pain, pathological fracture, hypercalcemia and lethal complications. Using quantitative proteomics mass-spectrometry (MS) analysis of the secretome from patient-derived primary BC cells (PBCs) and bone-metastatic BC cells (BMBCs), we identified an unreported breast cancer (BC) cells–secreted microprotein, osteolysin, encoded by tropomyosin 3 pseudogene 9 (TPM3P9) and clinically associated with BC bone-tropism.