where mab is the number of pairs, a and b observed, w is the weight given to each observation, f(s) is the relative frequency of occurrence of any amino acids at grid point s, and gab(s) is the equivalent relative frequency of occurrence of amino acid a against base b. R and T are gas constant and absolute temperature, respectively. Here, we used a box of |x| = |y| =13.5Å and |z| = 6 Å and a grid interval of 3Å, which was determined by examining various intervals. In order to quantify the specificity, we evaluated Z-score by calculating energy against 50,000 random DNA sequences. Z-score is defined by (X - m)/s in the histogram, where X is sum of contact potential in a complex form, m is mean energy over 50,000 combinations and s is standard deviation of the energy. For example, Z-score of -3.0 means that there are potentially two DNA sequences that are better fit to the framework among 1,000 random sequences.