(Molecular biology) The first codon in the coding region of an mRNA molecule. In all known organisms the start codon is AUG (or ATG, if you're reading from DNA). The ribosome reads along an mRNA molecule until it reaches the first AUG codon, at which point it begins the process of translation by adding a methionine amino acid. Each successive codon is then read and translated into an amino acid according to the genetic code.