The codon present in the beginning of a functional gene is called initiation codon. It marks the message for a polypeptide chain. This initiation codon is AUG in most of the cases and it codes for methionine (met) in eukaryotes and for N-formyl methionine (f-met) in prokaryotes less often GUG also serves as initiation codon in bacterial protein synthesis and codes for valine. UAG, UGA and UAA are termination codons or non-sense codons.