SDC1 and plasma cell myeloma: Myeloma expansion is further regulated by the Wnt/β-catenin signaling pathway that is often over-activated in MM, partially driven by the autocrine release of Wnt ligands and/or by the (over-) expression of distinct molecules such as Syndecan-1 (CD138) or Leucine Rich Repeat Containing G Protein-Coupled Receptor 4 (LGR4) on cancer cells (44–46, 115).