[didipao]

This is a good point. I just assumed didipao had access to the gene sequence from this statement:

I know which gene the peptides come from. In some cases I sequenced 2 versions of the peptide: one corresponds to a canonical protein (100% blast) and the other version of the peptide differs by one single amino acid. For most cases I did not find any known SNP in dbSNP that corresponds to the variation I found in the peptide. So I suppose that either I have a new DNA mutation (I have to confirm by sequencing) or a "post-DNA" modification such as RNA editing....I also plan to sequence the RNA.

In some cases I have 2 versions of the peptide, and the variation is reported in dbSNP. I still need to confirm by sequencing the DNA or other methods. So, I supposed that in such cases I can used the method suggested by gringer.