Serum bone profile and cathepsin K expression as a prognostic factor in patients with and without breast cancer metastasis B reast cancer is the most common malignancy among women and the leading cause of death. The majority of the cases are diagnosed at an advanced stage with a higher incidence of skeletal metastases [1][2][3]. Metastatic disease to the bone has been a crippling devastating complication of breast cancer, leaving patients bedridden or wheelchair-bound and victims of suffering from intolerable pain. The biological mechanisms leading to bone metastasis have been referred to as "vicious cycle" a complex network between cancer cells and the bone microenvironment [4]. Besides clinical and imaging techniques, biochemical tests play a vital role in the assessment and differential diagnosis of bone metabolic disorder in breast cancer [5]. These biochemical indices are non-invasive, comparatively low-cost and when applied and interpreted correctly, it is a great tool among the diagnostic and therapeutic assessment of metabolic bone Objectives: Breast cancer is the most frequently diagnosed cancer and the leading cause of cancer-related death among women worldwide. Breast cancer bone metastasis is associated with skeletal events, including acute fractures, compression of the spinal cord, surgery and radiotherapy to the spine, as well as bone pain and hypercalcemia, resulting in decreased mobility and diminished quality of life. Greater understanding of the bone metastasis pathophysiology will highly likely to lead to the discovery of an effective treatment option. This study aims to test whether the serum bone profile and expression level of cathepsin K (CTSK) in breast carcinoma is associated with metastasis. Methods: In this study, 116 participants, 58 patients who had been diagnosed with breast cancer (n=22 without metastasis and n=36 with metastasis) and 58 healthy controls were included. Serum biochemical profile and immunostaining of CTSK in the breast carcinoma were investigated.