Xanthine oxidoreductase (XOR) has been implicated in the process of oncogenesis either directly because it is able to catalyze the

metabolic activation of carcinogenic substances or indirectly through the action

of XOR- derived reactive oxygen and nitrogen species. XOR is strictly modulated at the transcriptional and post- translational levels, and its expression and activity are highly variable in cancer. Xanthine oxidoreductase (XOR) expression has been negatively associated with a high malignity grade and a worse prognosis in neoplasms of the breast, liver, gastrointestinal tract, and kidney, which normally express a high level of XOR protein. However, the level of XOR expression may be associated with a worse outcome in cancer of low XOR- expressing cells, in relation to the inflammatory response elicited through the tissue damage induced by tumor growth.