The three most frequently used restriction enzymes are Eco RI, Bam HI and Hind III. Each enzyme catalyses cleavage at the defined base sequence. All enzymes are lyophilised and contain 1500 units. One unit is defined as the amount of enzyme required to digest 1,0 µg of lambda DNA in 60 minutes at 37 °C in a total reaction mixture of 50 µl.