Breast cancer typically develops when glandular tissue of the breast becomes mutated. Over time, breast cancer cells gain the ability to invade other cells and tissues throughout the body. In some cases, breast cancer spreads to tissue within and around the shoulder leading to a number of symptoms. The development of joint pain is a common symptom of breast cancer metastasizing to the shoulder. As the breast cancer cell colonize tissue in the shoulder they attack neighbouring heathy cells and cause tissue damage that can lead to pain a study published in “Current Oncology” in 2006 indicates that breast cancer metastasis can lead to the development of cancerous lesions of the humerus, the upper arm bone. As the cancer forms growth on the head of the humerus, where the arm meets the shoulder joints, the tumor can interfere with normal joint functioning, leading to pain when moving the shoulder. Breast cancer patients experiencing unexplained shoulder pain should seek medical attention to assess the possibility of breast cancer to shoulder joints.