Note that in Gusfield's book, the entries M ( i , j ) {\displaystyle M(i,j)} and PAM n ( i , j ) {\displaystyle {\text{PAM}}_{n}(i,j)} are related to the probability of the i {\displaystyle i} th amino acid mutating into the j {\displaystyle j} th amino acid.[1] This is the origin of the different equation for the entries of the PAM matrices.