For example, circulating cancer cells can be attracted to bone by osteoblasts (OBs) or their progenitors, mesenchymal stem cells (MSCs), by expression of proteins, such as integrins, chemokines, Notch, nestin, and osteopontin by these cells12, and also via interactions between RANKL on OBs/MSCs and RANK expressed by cancer cells13,14. Here, SPP1 is linked to cancer.