S100A4 and breast cancer: Recently, modified views of our understanding of tumor bone metastasis have added dormancy and reactivation stages prior to the vicious cycle stage between tumor cells and osteoclasts.5 Because osteoclasts may also have a role in the reactivation process, it would be intriguing to investigate the role of S100A4 in the regulation of dormancy induction or reactivation of breast cancer cells in the bone-metastatic niche.