CXCL12 and breast cancer: Produced by CTCs, SDF-1 may help establish a migratory system that causes CTCs to localize among endothelial cells and osteoblasts that produce SDF-1 in the bone marrow, and chemokines may directly stimulate the proliferation of CTCs.188 CXCR4 may not only be responsible for invasion but may also be critical for the growth of micrometastases in some cancers.187 Wu et al.189 revealed that ER-regulated secretory protein (SCUBE2) contributes to the bone tropism of luminal breast cancer by modulating osteoblast differentiation and immune-suppressive osteoblastic niches.