About 35 selenoproteins have been identified and characterized, though many have roles that have not yet been fully elucidated. Selenocysteine represents the 21^st amino acid which is encoded by the UGA triplet in selenoproteins mRNA. Incorporation of selenocysteine in selenoproteins is rather complex but has been widely elucidated in prokaryotes. Four gene products (SELA, SELB, SELC, and SELD) and a specific stem-loop secondary structure which is termed selenocysteine insertion sequence (SECIS element) are required. However, the biosynthetic pathway of selenoproteins in eukaryotes may proceed by different routes in such aspects as the position and structure of SECIS element, specific elongation factors and other RNA-RNA or RNA-binding protein factors interactions. Observations also showed that translation of UGA as Sec in mammalian cells was an inefficient process and the regulation of the same UGA codon existing in an identical position in mRNA serves different functions (stop codon and Sec codon) might be involve in this process.