Substrates with one SfiI site are cut less efficiently than substrates with two sites. ""SfiI exists in solution as a tetramer and it appears to interact with two copies of it's recognition sequence before it can cleave DNA. It's primary reaction is then to cut both strands at both sites in a concerted process. The two sites can be either on the same or different DNA molecules.""- from Lois M. Wentzell, Timothy J. Nobbs and Stephen E. Halford, Mol. Biol. 248:581-595 (1995). ""The SfiI restriction endonuclease makes a four strand DNA break at two copies of it's recognition sequence.