Restriction enzymes cut at specific sequences named palindromic sequences. These are unique as they read the same in opposite directions. A specific palindromic sequence is complementary to a specific restriction enzyme. If the DNA fragment that is required to be cut out is flanked by two palindromic sequences these are cut forming sticky ends. These are necessary if the DNA fragment was then ligated into a plasmid/ vector. It is important that the target DNA/ vector is cut with the same restriction enzyme so complementary sticky ends are produced.