Name two reasons why it is impossible to determine a gene's nucleotide sequence from the amino acid sequence of the polypeptide?

Question

I can only think of one reason, which is because different codons can specify the same amino acids. However I am having trouble thinking of another reason.

Answer 1

Another answer that comes to mind is the presence in eukaryotic genomes of introns: sequences of DNA that are not coded into amino acids. The eukaryotic cell has a mechanism to remove these sequences from transcribed mRNA before the mRNA is read by ribosomes to assemble the amino acids to make a protein. Thus, knowing the amino acid sequence of a protein doesn't allow you to reconstruct the entire gene sequence as it exists in DNA.