In this study, we first found that Siglec15 protein levels were higher in tumor tissues than in normal tissues, and elevated Siglec15(SA)— but not Siglec15(TA) and Siglec15(whole)—correlated with poor prognosis in COAD and MMR-p COAD and served as an immune suppressor by suppressing CD8+ T cell, but not CD4+ T cell. This evidence concerns the gene SIGLEC15 and neoplasm.