restriction endonucleases, also known as restriction enzymes are enzymes that cleave the sugar-phosphate backbone of the dna at specific nucleotides sequences that are usually four to six nucleotides long. studies performed by horton and colleagues have shown that the mechanism by which these enzymes cleave the dna involves base flipping as well as bending the dna and the expansion of the minor groove. in 2006, horton and colleagues, x-ray crystallography evidence was presented showing that the restriction endonuclease hinp1i utilizes base flipping in order to recognize its target sequence. this enzyme is known to cleave the dna at the palindromic tetranucleotide sequence g↓cgc.