EBAG9 is an estrogen-responsive gene located at chromosome 8 that functions as a tumor-promoting factor with dual roles in cell death and immune evasion. Primary function: EBAG9 may participate in suppression of cell proliferation and induce apoptotic cell death through activation of ICE-like proteases 1. However, paradoxically, EBAG9 overexpression promotes tumor growth in vivo through its coiled-coil region, likely via intracellular associations with unidentified binding partners 2. Mechanism: EBAG9 is identical to RCAS1, a cancer cell surface antigen implicated in immune escape 1. The protein inhibits release of cytolytic enzymes from cytotoxic T lymphocytes and inversely correlates with intratumoral T cell infiltration 34. Disease relevance: EBAG9 is upregulated in ischemic acute kidney injury within injured proximal tubules displaying proapoptotic and proinflammatory phenotypes 5. In cancer, EBAG9 expression correlates with advanced pathologic stage, poor prognosis, and metastasis across testicular, prostatic, and breast cancers 678. Clinical significance: EBAG9 gene amplification occurs in 21-45% of primary breast cancers, with exclusive EBAG9 gain associated with early-stage disease 8. EBAG9 silencing in engineered T cells enhances cytolytic competence and tumor eradication without increasing cytokine release syndrome risk 4, suggesting therapeutic potential for adoptive immunotherapy.