A set of residues considered to be mutable. This should be a subset of the
"High Entropy Residues" set. Typically it is equal to the "High Entropy
Residues", however, if one wishes not to mutate Aspartates then this residue
can be removed from this set.
Mutations will be introduced in the order listed here. For example, using the default set of "KEQD" and a cluster containing two K's and two and two E's that are otherwise equivalent, mutation of the K's will be preferred as this residue is listed first in this set.