The sequence of amino acids in a protein is determined by the series of codons starting from a fixed translation initiation codon. AUG and GUG are the major translation start codons of prokaryotic genes. AUG is almost always the translation start codon of eukaryotic genes. The bacterial start AUG and GUG codons specify a modified form of methionine, whereas AUG or GUG codons internal to reading frames specify methionine or valine, respectively.