The first codon in this case is Cu g. But because Cua and Cu g basically code for the same exact amino acid, leucine, the first amino acid is not changed. But look what happens to the next. Because we insert this single nucleotide into our sequence, it shifts all the other nucleotides and that changes the codons that are read by the ribosome. So in this case, the second codon was GCU. In this case, because we had a shift in the reading frame, the second codon is AGC, which is different than GCU. Then this one is CGC.