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