DNA vs. Protein searches
DNA is composed of 4 characters: A,G,C,T It is anticipated that on the average, at least 25% of the residues of any 2 unrelated aligned sequences, would be identical.
Protein sequence is composed of 20 characters (aa). The sensitivity of the comparison is improved. It is accepted that convergence of proteins is rare, meaning that high similarity between 2 proteins almost always means homology.