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{{Further|List of genetic codes}}
The genetic code was once believed to be universal:<ref name="evolve1"/> a codon would code for the same amino acid regardless of the organism or source. However, it is now agreed that the genetic code evolves,<ref name="evolve2"/> resulting in discrepancies in how a codon is translated depending on the genetic source.<ref name="evolve1">{{cite journal|last1=Osawa|first1=A|date=November 1993|title=Evolutionary changes in the genetic code|url=https://pubmed.ncbi.nlm.nih.gov/8281749/|journal=Comparative Biochemistry and Physiology|volume=106|issue=2|pages=489–94|doi=10.1016/0305-0491(93)90122-l|pmid=8281749|access-date=2020-12-05|archive-date=2020-12-06|archive-url=https://web.archive.org/web/20201206173716/https://pubmed.ncbi.nlm.nih.gov/8281749/|url-status=live}}</ref><ref name="evolve2">{{cite journal |vauthors=Osawa S, Jukes TH, Watanabe K, Muto A|date=March 1992|title=Recent evidence for evolution of the genetic code|journal= Microbiological Reviews|volume=56|issue=1|pages=229–64|doi=10.1128/MR.56.1.229-264.1992|pmc=372862|pmid=1579111}}</ref> For example, in 1981, it was discovered that the use of codons AUA, UGA, AGA and AGG by the coding system in mammalian mitochondria differed from the universal code.<ref name="evolve1"/> Stop codons can also be affected: in [[Ciliate|ciliated protozoa]], the universal stop codons UAA and UAG code for glutamine.<ref name="evolve2"/>{{efn|group=note|''[[Euplotes octacarinatus]]'' is an exception.<ref name="evolve2"/>}} Four novel alternative genetic codes (numbered here 34–37) were discovered in bacterial genomes by Shulgina and Eddy, revealing the first sense codon changes in bacteria.<ref name="Shulgina">{{cite journal |last1=Shulgina |first1=Yekaterina |last2=Eddy |first2=Sean R. |date=9 November 2021 |title=A computational screen for alternative genetic codes in over 250,000 genomes. |journal=eLife |volume=10 |doi=10.7554/eLife.71402|doi-access=free |pmid=34751130 |pmc=8629427 }}</ref> The following table displays these alternative codons.
