During depression there are increased levels of radical nitrogen species (RNS) and reactive oxygen species (ROS) such as peroxide, and the activities of pro-oxidant enzymes such as xanthine oxidase (XO) are also increased, nitric oxide (NO) levels are changed and there are indications of oxidative and nitrosative stress (O&NS) damage to fatty acids and DNA [39]. The gene discussed is XDH; the disease is major depressive disorder.