GNLY (granulysin) is an antimicrobial protein expressed by natural killer cells and activated T lymphocytes that exhibits broad-spectrum cytolytic activity against microbes and tumors 1. The protein functions as an effector molecule in various cytotoxic T cell subsets, particularly in CD8+ effector memory T cells and CD4+ effector T cells 23. In immune surveillance, GNLY+ T cells serve as key markers of cytotoxic function - GNLY+CD8+ effector memory T cells show high clonal expansion and distinct functional signatures across different age groups 4, while GNLY+ CD4+ T cells are associated with enhanced immune responses in conditions like checkpoint inhibitor colitis 5. The protein plays crucial roles in antimicrobial immunity, with GNLY+ T cell populations expanded during viral infections like COVID-19, where intensive expansion of CD4+ and CD8+ effector-GNLY subsets is associated with disease convalescence 2. Additionally, CX3CR1+GNLY+ central memory CD8+ T cells serve as potential surrogate markers for viral resolution in hepatitis B infection 6. Clinically, granulysin expression levels in NK cells correlate with favorable cancer outcomes, suggesting its utility as both a diagnostic marker and therapeutic target 17.