Codon is the group of three consecutive nucleotides within the messenger RNA (mRNA) which encodes a message to initiate translation, to incorporate the specific amino acid into the growing polypeptide chain, or to stop the translation. The sequence of the codons in the mRNA unambiguously defines primary structure of the final protein. Of course, codons in the mRNA were also present in genomic DNA, but the sequence might be interrupted by introns.