CD44 and neoplasm: Several receptors and their ligands have been implicated in these intercellular interactions; for example CXCR-4 (receptor on HSC)/SDF-1 (ligand secreted by osteoblastic lineage cells) axis represents one of the most extensively investigated while hyaluronan and osteopontin (ligands expressed in the bone)/CD44 (receptor and putative marker for CSC) binding suggest a possible adhesive interaction for circulating tumor cell arrest.