Accepted point mutation substitution

A sophisticated model of protein evolution was developed by Dayhoff et al. (1978) who measured the frequency with which each amino acid was replaced by every other in sets of closely related sequences. This was converted into a Markov model, which was used to generate the probability of any amino acid being substituted by any other or remaining unchanged after different amounts of evolution. The amount of evolution was measured in PAMs (point accepted mutations), which are mean numbers of substitutions per 100 residues. This is, of course, the same... 

PAM Point Accepted Mutation. The PAM matrix is a frequency table representing substitution rates for closely related proteins at the particular evolutionary distance represented by multiple sequence alignments. 

---