1. The sequence length, i.e. the number of residues of the peptide. 2. The isoelectric point, computed by using the iterative method described by Tiengo et al. [24] . 3. The amino acid frequencies (24 features), calculated as the occurrence of each amino acid along the peptide; the four ambiguous amino acid B (asparagine or aspartic acid), X (unspecified or unknown amino acid), Z (glutamine or glutamic acid) and J (leucine or isoleucine) have also been considered.