The effect of syndecan-1 may partly depend on its endogenous level in the specific cell type studied as well as the ratio between membrane bound and shed syndecan, competing for ligand binding as shown in a breast cancer cell line, where overexpression of wild type syndecan-1 increased proliferation, but overexpression of constitutively shed syndecan-1 inhibited it [46]. This evidence concerns the gene SDC1 and breast cancer.