Homology or identity can be determined by comparing a position in {TAIL ~ each sequence} which may be aligned for {HEAD ~ purposes} of comparison 
Homology or identity can be determined by comparing a position in { TAIL ~ each sequence } which may be aligned for { HEAD ~ purposes } of comparison
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD OTH OTH