ii) Continuous and non-directional (Grantham's distance): A value from 5 (most similar, leucine-isoleucine) to 215 (most dissimilar, cysteine-tryptophan) was assigned to each amino acid substitution mapping it to Grantham's distance matrix. This distance depends on three properties: composition, polarity, and molecular volume; and is independent of the directionality of the change (e.g. leucine > isoleucine is the same as isoleucine > leucine).