ATG or AUG: The codon for the methionine; the translation initiation codon. generally, protein translation can only start at the methionine codon (although this codon might be found elsewhere within the protein series as well). In eukaryotic DNA, the series is ATG; in RNA it is AUG. Generally, the first AUG in the mRNA is the point at which the translation begins, and an open reading frame follows - that is the nucleotides taken three at the time will code for the amino acids of protein, and a stop codon will be found only when protein coding region is complete.