The sequence of a hypothetic protein for illustration of derivation of the feature vector of a protein. Sequence index indicates the position of an amino acid in the sequence. The index for each type of amino acids in the sequence (A or E) indicates the position of the first, second, third, ... of that type of amino acid (e.g., the position of the first, second, third, ..., A is at 1, 3, 4, ...). A/E transition indicates the position of AE or EA pairs in the sequence.