According to the hydropathy profile of amino acids, the amino acids can be classified into three groups (Nei and Kumar, 2002; Liu and Wang, 2006) : internal group (F, I, L, M, V), external group (D, E, H, K, N, Q, R), and ambivalent group (S, T, Y, C, W, G, P, A). The amino acid of internal group tends to occur in the inner side of the protein's spatial structure, while the amino acid of external group tends to appear at the surface. In order to characterize the hydropathicity of a protein primary structure, we defined a primary protein sequence as a symbolic sequence including three letters according to the following rule: