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An amino acid used in several enzymes (glutathione peroxidase, tetraikiodothyronine 5' deiodinase and formate dehydrogenase for example). Selenocystein has a structure similar to cysteine, but with an atom of selenium taking the place of the usual sulfur. Unlike other amino acids used in biological protein, however, it is not coded for directly in the genetic code. It is coded for by a UGA codon, which is normally a stop codon but in these exceptional cases is modified by a subsequent sequence in the mRNA molecule encoding the enzyme. When cells are grown in the absence of selenium protein translation terminates normally at the UGA codon instead, resulting in a truncated and nonfunctional enzyme. The primary and secondary structure of selenocysteine tRNAs differ from standard tRNAs in several respects, most notably an eight base pair acceptor stem, a long variable region arm, and substitutions at several well-conserved base positions.