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Grantham Score
The Grantham Score is a measure of physico-chemical difference between amino acids. It was first proposed by R. Grantham in 1974 in the journal Science.

When there is a deviation from the reference sequence of a gene the change in base can result in a change in amino acid after translation. In human medicine it can be important to ascertain if this mutation is likely to be pathogenic or not. One of the many methods that can be used to try to deduce the likely pathogenicity of an amino acid change is the Grantham Score.

The formula, used to compare two amino acids, takes into account several properties including their polarity and physical size. The larger the Grantham Score the more different two amino acids are and the more likely this change is to produce a change in function of the resultant protein. This is because the function of a protein is highly dependent upon its physical properties and if these properties change, due to one of its component amino acids being swapped with another for example, then the function of that protein can change.

http://www.sciencemag.org/content/185/4154/862.abstract

Science, 1974: Vol. 185 no. 4154 pp. 862-864 Amino Acid Difference Formula to Help Explain Protein Evolution