Selenocysteine

Selenocysteine is a rare amino acid that is incorporated into a few proteins using the codon UGA. Normally UGA is stop codon, but the proteins that use UGA for selenocysteine usually have mRNA with significant secondary structure around the UGA. This may cause the ribosome to not recognize the UGA as a stop codon, allowing the selenocysteinyl-tRNA to incorporate the amino acid at that point.

See also: Amino Acids, Glutathione Peroxidase, Genetic Code