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Dayhoff et al.  presented a method
for estimating the matrix M from the observation of 1572 accepted
mutations between 34 superfamilies of closely related sequences.
Their method was pioneering in the field.
Nowadays we are able to estimate M by more accurate
and better founded methods [8,4].
A Dayhoff matrix
(in honour of Margaret O. Dayhoff) is a matrix,
computed from a 250-PAM mutation matrix, used for the standard dynamic
programming method of sequence alignment.
The Dayhoff matrix entries are related to M250 by