SELENOS and cancer: Se is a vital trace element involved in many biological processes that are mediated by selenoproteins.30 An inverse relation between Se exposure and cancer risk was suggested by early studies, but subsequent controlled trials reported that selenium supplementation does not reduce the risk of cancer and may even increase it for some cancer types.31 These observations may be explained by our findings that tumor tissues bear higher Se concentration than their paired normal lung tissues, and Se may have a role in promoting carcinogenesis.