S1 Data -

<div><p>Breast cancer (BRCA) is among the most prevalent cancers and is responsible for numerous patient fatalities. Immunotherapy has emerged as a promising approach to cancer treatment. Recent studies have identified Siglec-15 as a novel immune target that plays a crucial role in tumor immune evasion, suggesting its potential significance in BRCA. We utilized databases such as TCGA to investigate the relevance of SIGLEC15 in BRCA. The expression of the Siglec-15 protein in 74 breast cancer patients was detected using immunohistochemistry, and its association with clinicopathological features and overall survival was evaluated. The co-expression of Siglec-15, CD68, CK, and CD8 in BRCA tissues was identified through multiplex immunofluorescence staining. Our study revealed that SIGLEC15 expression in BRCA was significantly elevated compared to adjacent normal tissues. Kaplan-Meier analysis identified SIGLEC15 as a prognostic protective factor. According to the receiver operating characteristic curve analysis, SIGLEC15 could predict the luminal subtype of BRCA. Enrichment analysis demonstrated that SIGLEC15 involves various biological pathways, including immunity, metabolism, tumors, and infectious diseases. Correlation analysis revealed an association between SIGLEC15 expression and immune infiltration in BRCA. We also confirmed that the Siglec-15 protein is expressed in cancer cells, tumor-infiltrating T cells, and macrophages in BRCA tissues, significantly higher levels than in normal breast tissues. Consequently, SIGLEC15 correlates with tumor immune infiltration, molecular subtypes, and BRCA progression and prognosis. However, further research is required to elucidate the role of SIGLEC15 in breast cancer.</p></div>