Selenium, with the symbol "Se", is a chemical element between metal and nonmetal. Selenium is an essential trace element for human body. Selenium usually exists in the form of compounds, which can be divided into three categories: inorganic selenium, organic selenium and artificial synthesis. Inorganic selenium compounds mainly include selenite, sodium selenite, sodium selenite, selenium disulfide, selenium sulfide, selenium dioxide and so on. Organic selenium compounds mainly include racemic selenomethionine, selenic acid, selenium polysaccharide, seaweed selenium oligosaccharide and so on. Selenium compounds are mainly synthesized by selenium yeast and selenium-enriched wheat. -External ribonucleic acid plus selenium oral liquid