QSOX1 (quiescin sulfhydryl oxidase 1) is an extracellular enzyme that catalyzes the oxidation of sulfhydryl groups in peptide and protein thiols to disulfides, reducing molecular oxygen to hydrogen peroxide 1. This enzyme plays a critical role in disulfide bond formation in extracellular proteins and is required for normal incorporation of laminin into the extracellular matrix, thereby facilitating cell-cell adhesion and migration 1. QSOX1 functions as a cellular pro-oxidant that disrupts redox homeostasis by inhibiting activation of the antioxidant transcription factor NRF2 through promotion of EGFR endosomal trafficking and degradation 2. In cancer contexts, QSOX1 is overexpressed in multiple tumor types including breast, pancreas, prostate, lung, hepatocellular carcinoma, and colorectal cancer, where it facilitates tumor cell invasion and migration at the tumor-stroma interface 1. The enzyme promotes cancer immune escape by upregulating PD-L1 expression and excluding CD8+ T cells from dormant cancer stem cell niches 3. QSOX1 also enhances drug resistance mechanisms, including gefitinib resistance through RNA m5C modification pathways 4 and anti-CTLA-4 therapy resistance by promoting regulatory T cell-mediated immune tolerance 5. These findings position QSOX1 as both a prognostic biomarker and potential therapeutic target in cancer treatment.