Hi there,

X is often used to highlight the presence of an unknown amino acid.

It may happen due to multiple sequences showing different amino acids or if the original sequence's quality was too poor to attribute a nucleotide with certainty.

Here is a list of all the letters used for nucleotides and amino acids abbreviation.

https://www.genome.jp/kegg/catalog/codes1.html

Hope this helps!

Thanks a lot@Francesco Martoni

Thank you Rob Keller

How we can fix problem of X in protein and y in nucleotide?

Which amino acid we choose in place of X?