QSOX2, also known as Quiescin Sulfhydryl Oxidase 2, is an enzyme that plays a key role in the oxidative folding of proteins, especially those with multiple disulfide bonds. Located mainly in the endoplasmic reticulum (ER) of cells, QSOX2 is essential for maintaining protein structure and function by catalyzing disulfide bond formation between cysteine residues. This activity supports proper protein folding, stability, and secretion, acting as a thiol oxidase that transfers electrons from reduced cysteines to molecular oxygen.