It is hypothesised that tumour cells either remain dormant or are stimulated to proliferate into overt metastases dependent on which niches they interact with: Interactions between CXCR4, found on metastatic tumour cells, and CXCL12, expressed by osteoblasts, mesenchymal stem cells (MSCs) and adventitial reticular cells (Sugiyama et al. 2006) are a key component in homing and adhesion of tumour cells to the bone metastatic niche. Here, CXCR4 is linked to neoplasm.