The start codon is the first codon of a messenger RNA (mRNA) transcript that is translated by a ribosome. It is a sequence of three nucleotides that codes for a specific amino acid, which is usually methionine in eukaryotes and archaea, and N-formylmethionine in bacteria, mitochondria, and plastids
. The standard start codon for translation of a gene is "AUG," which encodes the amino acid methionine and establishes the reading frame for the protein
. The start codon is often preceded by a 5' untranslated region (5' UTR), which includes the ribosome binding site in prokaryotes
. Alternative start codons, such as GUG and UUG, are different from the standard AUG codon and are found in both prokaryotes and eukaryotes
