S100A6 and breast cancer: Doxorubicin treatment induces the expression of miR-21-5p in MSCs and in mesenchymal stem-cell-derived exosomes, leading to the induction of S100A6 in the breast cancer cells (BCs). Silencing of miR-21-5p expression in MSCs and MSC exosomes abolished the resistance of BCs to doxorubicin, indicating an exosomal miR-21-5p regulated the role of S100A6 in chemoresistance both in vitro and in vivo.