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In classical genetics, the STOP codons were given names - UAG was amber, UGA was opal, and UAA was ocher. These names were originally the names of the specific genes in which mutation of each of these stop codons was first detected. Translation starts with a chain initiation or START codon, but unlike STOP codons these are not sufficient by themselves to begin the process; nearby initiation sequences are also required to induce transcription into mRNA and binding by [[ribosome|ribosomes]]. The most notable of these is AUG, which also codes for methionine. CUG and UUG, and in prokaryotes GUG and AUU, will also work.
Numerous variations on the standard genetic code are found inside [[mitochondrion|mitochondria]], energy-burning organelles that were probably derived from symbiotic bacteria. The [[Ciliophora]] or ciliate protozoa also show some variation in the genetic code: UAG and often UAA code for Glutamine, a variant also found in some [[green algae]], or UGA codes for Cysteine. One more variant is found in some species of the [[yeast]] ''Candida'', but interestingly not in all, where CUG codes for Serine. There are also a few "non-standard" amino acids which are substituted for some stop codons in some species of [[bacteria]] and [[archaea]]; UGA can code for [[selenocysteine]] and UAG can code for [[pyrrolysine]]. Other non-standard amino acids and codon interpretations may be present but currently unknown.
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