Selenium and selenocysteine: roles in cancer, health, and development

•Selenocysteine is the 21st amino acid in the genetic code.•Selenoproteins are largely responsible for the many health benefits of selenium.•Some selenoproteins exhibit a split personality in preventing and promoting cancer.

The many biological and biomedical effects of selenium are relatively unknown outside the selenium field. This fascinating element, initially described as a toxin, was subsequently shown to be essential for health and development. By the mid-1990s selenium emerged as one of the most promising cancer chemopreventive agents, but subsequent human clinical trials yielded contradictory results. However, basic research on selenium continued to move at a rapid pace, elucidating its many roles in health, development, and in cancer prevention and promotion. Dietary selenium acts principally through selenoproteins, most of which are oxidoreductases involved in diverse cellular functions.