Topic summary
Start codon
The start codon is the first codon of a messenger RNA (mRNA) transcript translated by a ribosome. The start codon always codes for methionine in eukaryotes and archaea and N-formylmethionine (fMet) in bacteria, mitochondria and plastids.
The start codon is often preceded by a 5' untranslated region (5' UTR). In prokaryotes this includes the ribosome binding site.