GGT5 (gamma-glutamyltransferase 5) is a cell surface enzyme that cleaves the gamma-glutamyl bond of extracellular glutathione and glutathione conjugates, including leukotriene C4 (LTC4) and S-geranylgeranyl-glutathione (GGG) 1. This proteolytic activity maintains extracellular cysteine levels essential for intracellular glutathione synthesis 2. GGT5 converts extracellular LTC4 to LTD4 during inflammatory responses 1 and acts as a negative regulator of GGG bioactivity, establishing GGG gradients in lymphoid tissues that position P2RY8-positive lymphocytes at germinal centers and restrict their chemotactic migration 34. In cancer pathology, GGT5 emerges as a significant prognostic biomarker. Elevated GGT5 expression in gastric cancer correlates with poor overall survival and is an independent risk factor for unfavorable prognosis 56. GGT5 facilitates cancer cell migration and invasion through epithelial-mesenchymal transformation 6. In colorectal adenocarcinoma, GGT5 is part of lipid metabolism-based prognostic signatures 7. Notably, GGT5 expression in stromal cells correlates with memory CD8+ T cell infiltration and reduced immunotherapy response in gastric cancer 8. Additionally, GGT5 is upregulated in severe asthma eosinophils through type 2 cytokine signaling and is counterregulated by retinoic acid 9.