Selenium (Se) is an essential micronutrient due to its requirement for biosynthesis and function of the 21st amino acid, selenocysteine (Sec). This amino acid is typically found in the active sites of a small number of selenoproteins in all three domains of life: archaea, bacteria and eukaryotes . Biosynthesis of Sec and its cotranslational insertion into polypeptides require a complex molecular machinery that recodes in-frame UGA codons, which normally function as stop signals, to serve as Sec codons . Although the occurrence of selenoprotein genes is limited, the Sec UGA codon has become the first addition to the universal genetic code since the code was deciphered 40 years ago .