This occurs when the heparan sulfate proteoglycan syndecan-1 on the surface of myeloma cells binds to OPG, leading to internalization and degradation of the OPG.(15) Because syndecan-1 is expressed at high levels on most myeloma tumor cells, the binding and degradation of OPG may be a substantial contributor to the bone-degrading phenotype of myeloma. This evidence concerns the gene CD44 and plasma cell myeloma.