Instead of attempting to determine this relationship, the values of M {\displaystyle M} are calculated for a short time frame, and the matrices for longer periods of time are calculated by assuming mutations follow a Markov chain model.[9][10] The base unit of time for the PAM matrices is the time required for 1 mutation to occur per 100 amino acids, sometimes called 'a PAM unit' or 'a PAM' of time.[3] This is precisely the duration of mutation assumed by the PAM1 matrix.