Selenium (Se) is required in selenocysteine (Sec, U), the 21st genetically encoded amino acid and the most potent physiological nucleophile. Humans possess 25 selenoprotein genes expressed in tissue-dependent distributions with functions that are particularly important in brain and neuroendocrine tissues. Selenoenzymes prevent and reverse oxidative damage to lipids and proteins, support immune functions, assist in regulating calcium and thyroid hormone metabolism, tubulin polymerization, protein folding, regulate Se transport in the body, and one is required for Sec synthesis (Ralston & Raymond 2018).