iii) Continuous and directional (Epstein's coefficient of difference): A value from 0 (most similar) to 1 (most different) was assigned to each amino acid substitution mapping it to Epstein's coefficient of difference matrix. This coefficient depends on the polarity and size of the replaced amino acids and takes into account directionality (e.g. leucine > tyrosine is 0.28 and tyrosine > leucine is 0.22).