{HEAD ~ a substitution} of {TAIL ~ the amino acid} which occupies the position with a different amino acid, 
of
{ HEAD ~ a substitution } of { TAIL ~ the amino acid } which occupies the position with a different amino acid ,
HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH