AUG or methionine codon has dual function. Duality in the codon function is known in the use of AUG in initiation of translation and methionine insertion into internal protein positions. When AUG is at the beginning of prokaryotic message (starting codon), it will code for the incorporation of N formylmethionine (fMet) whereas AUG within the message will code for normal methionine (Met).