In spite of successful treatment of the primary tumour, breast cancer patients may die of their disease because tumours often reappear in other sites of the body. While there are adequate methods to deal with the tumour in the breast, effective treatment for the spreading tumour outside the breast has not yet been discovered. Osteoclasts are bone cells that are normally responsible for dissolving the bone substance. When cancer cells reach the bone and start to grow in it, they exploit osteoclasts to destroy the bone, make room for cancer cells to expand and supply cancer cells with growth factors. Patients who develop secondary tumours in their bones need treatment that stops osteoclast action such as bisphosphonates. However, bisphosphonates are less effective in cancer patients than in other conditions. We are studying why osteoclasts are resistant to bisphosphonates in the presence of breast cancer cells, with the goal to develop more efficient treatment against excessive osteoclasts action, which consequently will limit and/or reverse bone destruction and tumour growth at the bone.