backtranseq takes a protein sequence and makes a best estimate of the likely nucleic acid sequence it may have come from.
Here is a sample session with backtranseq. Note that this is a human protein, so the default (human) codon frequency file is used:
% backtranseq Back translate a protein sequence Input sequence: sw:opsd_human Output sequence [opsd_human.fasta]:
Here is a session using a drosophila sequence and codon table:
% backtranseq -cfile Edrosophila.cut Back translate a protein sequence Input sequence: sw:ach2_drome Output sequence [ach2_drome.fasta]: