text: A rare-cutter enzyme is a restriction enzyme with a recognition sequence which occurs only rarely in a genome.  An example is NotI, which cuts after the first GC of a 5-GCGGCCGC-3 sequence; restriction enzymes with seven and eight base pair recognition sequences are often also called rare-cutter enzymes (six bp recognition sequences are much more common).
  For example, rare-cutter enzymes with 7-nucleotide recognition sites cut once every 4 bp (16,384 bp), and those with 8-nucleotide recognition sites cut every 4 bp (65,536 bp) respectively. They are used in top-down mapping to cut a chromosome into chunks of these sizes on average.