At least 16 organisms from a diverse array of evolutionary lineages deviate from nature's standard code in the amino acid "meaning" they assign to specific codons. Many species of the green alga Acetabularia, for example, translate the standard "stop" codons UAG and UAA as the amino acid glycine. To Candida fungi, the RNA codon CUG, which normally means leucine, instead specifies serine.
The existence of such variations demonstrates that the code can evolve and may provide clues about how it did. In all three domains of life, a nonstandard 21st amino acid, selenocysteine, is sometimes fabricated in response to the standard stop codon UGA. Selenocysteine is created by chemical tweaking of serine while that amino acid is still attached to its tRNA in the ribosome. In two domains (archaea and bacteria), a 22nd amino acid, pyrrolysine, is produced in the same manner, in response to the standard stop codon UAG.