What is Rare-cutter enzyme?

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.

For example, rare-cutter enzymes with 7-nucleotide recognition sites cut once every 4 bp , and those with 8-nucleotide recognition sites cut every 4 bp respectively. They are used in top-down mapping to cut a chromosome into chunks of these sizes on average.