The extent to which an amino acid in a protein is accessible to the surrounding solvent is highly dependent on the type of amino acid. In general, hydrophilic amino acids tend to be near the solvent accessible surface, while hydrophobic amino acids tend to be buried in the core of the protein. To measure this effect, several solvent exposure measures have been proposed [1-7], and one of these is the contact number measure (CN) [7]. The CN of a residue is the number of Cα atoms in a sphere centered at the Cα atom of the residue in question (Figure 1). The CN of all residues of a protein is called the CN vector. The CN vector is well conserved and can be predicted with high accuracy [8].