Margaret Dayhoff observed that protein sequences undergo variation during evolution according to certain patterns. She noted that:
– Amino acids were not replaced at random but were altered with specific preferences. For example, amino acids of similar physico-chemical characteristics were preferred, one over another.
– Some amino acids such as tryptophan, was generally not replaced by any other.
– Based on several homologous sequences, a point accepted mutation (PAM) matrix could be developed. This laid the first foundation for subsequent work on sequence comparisons using quantitative approaches.