Triple-negative breast cancer (TNBC) is recognized as the most aggressive form of breast malignancy. Although TNBC patients have a 5-year overall survival rate of 77%, the prognosis drops sharply to 12% once metastasis develops. The identification of biomarkers that accurately predict TNBC metastasis remains an urgent unmet need. In this study, we discovered a previously uncharacterized micropeptide, XLH-36, derived from the long non-coding RNA (lncRNA) C5orf66-AS1, which functions as a potent oncogenic factor in TNBC. Comparative sequence analyses across 101 species revealed that XLH-36 is highly conserved in humans and primates. Analysis of RNA-seq datasets from 1295 breast cancer patients, including 165 TNBC cases across multiple cohorts, demonstrated elevated XLH-36 expression in both breast cancer overall and TNBC specifically. Importantly, patients with lower C5orf66-AS1 expression, encoding XLH-36, exhibited a 20% survival advantage over 50 months compared with those with higher expression. Functional experiments showed that loss of XLH-36 significantly reduced tumor growth and metastasis in TNBC xenografts in vivo. Mechanistically, XLH-36 binds to Gemin4, retaining it in the cytoplasm and thereby disrupting its nuclear role in S100A4 mRNA splicing. This interaction triggers a compensatory upregulation of ICAM1, which facilitates epithelial-to-mesenchymal transition (EMT) and promotes metastatic dissemination. These findings establish XLH-36 as a critical regulator of TNBC metastasis and suggest its potential as both a biomarker and therapeutic target.
