ROS as a signaling molecule is necessary for cell proliferation, but at higher levels it can also induce cell death and prevent tumor growth. Together with these observations, our results reveal the complexity of tumorigenesis and the dual nature of oxygen. Oxygen can promote cancer initiation by increasing genomic instability through oxidative stress while also having the potential to inhibit the growth of established tumor cells through specific signaling mechanisms. The lessons from our current study may be applicable to human health. Supplemental oxygen is ubiquitously employed in clinical medicine because of its immediate benefits for energy production while the less apparent potential for genotoxicity can be neglected. Our work may provide a biological mechanism for important clinical observations such as the increased cancer risk of neonates exposed to supplemental oxygen or of Ki 20227 babies conceived through in vitro fertilization, which may be performed under 21% oxygen whereas the oxygen concentration in the uterus is 5- to 10-fold lower. Although it is difficult to control for the many variables that are associated with the requirement for oxygen therapy, our experiments reveal the potential genotoxicity of oxygen which may be more immediately relevant to the clinics. Thus, minimizing oxygen Toyocamycin exposure at early developmental stages such as in neonates or of in vitro fertilized oocytes prior to uterine implantation may potentially decrease the risk of cancer over a lifetime. Similarly, although antioxidants do not appear to have tumor suppressive effects in the general population, targeting individuals who have specific inherited cancer susceptibility syndromes due to defects in DNA repair to strategies of reducing oxygen exposure or antioxidant therapy may yield marked benefits. Lingzhi is a well-known anticancer fungus. Although it is the general name of Ganoderma species, the name Lingzhi usually means a single species G. lucidum, which is one of the most studied mushrooms in the world. Its anti-cancer effects are associated with not only the cytotoxic triterpenoids, but also the immunomodulating polysaccharides via the inhibition of DNA polymerase and post-translational modification of the Ras oncoprotein, or the stimulation of cytokine production. Both official Lingzhi species are rich in polysaccharides and possess multiple biological activities, such as antimicrobials, immunomodulation, and antitumor effect. Since G. sinense is traditionally used in the form of decoction, the water-soluble polysaccharides are also considered its major active ingredients, having immune-balancing, antioxidant and antitumor activities.