MPO and inflammatory bowel disease: Substantial studies have shown that a series of oxidative markers, such as ROS species, nitric oxide, 8-oxo-2'-deoxyguanosine (8-oxodG), and antioxidant or pro-oxidative proteins (e.g., catalase and myeloperoxidase), are altered in IBD and CRC, demonstrating a more severe oxidative microenvironment in IBD and CRC (Perse, 2013; Sies et al., 2017).