GSR and cancer: In the extracellular environment of a malignant tumor, metastasis of the cancer cells is initiated by releasing a massive amount of matrix-degrading enzymes, which creates a path for them to move on and substitute themselves into normal tissues [82], whereas in the intracellular region, glutathione is produced in large amount by the glutathione reductase enzyme (conversion of GSSG to GSH), which protects cells against reactive oxygen species (ROS).