{HEAD ~ The excised segment} can be {TAIL ~ as long as 28 nucleotides}, or more, and as little as one nucleotide 
be
{ HEAD ~ The excised segment } can be { TAIL ~ as long as 28 nucleotides } , or more , and as little as one nucleotide
HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH