When {HEAD ~ a position} in {TAIL ~ the compared sequences} is occupied by the same nucleotide base or amino acid, then the molecules are homologous, or identical, at that position 
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When { HEAD ~ a position } in { TAIL ~ the compared sequences } is occupied by the same nucleotide base or amino acid , then the molecules are homologous , or identical , at that position
OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH