But in this case, the third codon is UCG. Now CUG still codes for Leucine, but AGC codes for Serene and UCG codes for Serene once again. So these two codons code for the same exact amino acid. And notice that this, or this for that matter, is different than the sequence given here. We have serene and theory and we have Alanine and Arginine. So we see that the insertion of the cytosine nucleotide between the 7th and the 8th DNA nucleotide causes a shift in the mRNA reading frame.