Each amino acid residue in a protein sequence can be represented by a set of different states according to its features. For instance, its hydrophobicity feature can be marked by one of the following three states: ''polar'', ''neutral'', or ''hydrophobic'' [85] ; its solvent accessibility feature by one of the two: ''buried'' or ''exposed to solvent'', as predicted by PredAcc [35] ; its secondary structure feature by one of the three: ''helix'', ''sheet'', or ''coil'', as predicted by the method in [86] ; and so forth.