SIGLEC15 and cancer: In the tumor microenvironment, Siglec15-expressing cancer cells and tumor-associated macrophages facilitate tumor growth and metastasis by suppressing T-cell activation.18,40 Siglec15 knockout in macrophage-lineage cells also abrogates osteoclast formation, but the signaling mechanisms are not known, as the receptor for Siglec15 has not been identified.