Recently, a new promising solvent exposure measure, called half-sphere-exposure (HSE), has been proposed [9]. While the CN measure uses a single sphere centered at the Cα atom, the HSE measure considers two hemispheres. Two values, an up and a down value, are associated with each residue, corresponding to the upper and lower hemisphere. The geometry of the HSE construction is shown schematically in Figure 2. The up and down HSE values measure two fundamentally different environments of an amino acid, one of them corresponding to the neighbourhood of the side chain [9]. The HSE measure compares favorably with other solvent exposure measures in terms of computational complexity, sensitivity, correlation with the stability of mutants and conservation. An important advantage of the HSE measure is that it can be calculated from Cα -only or other simplified protein models. Therefore, it forms an attractive alternative to the use of the CN measure in protein structure prediction methods [10].